Passage Reference: It has been quite a while since man discovered fire. But it is only recently that he has learnt enough chemistry to think of improving it. Take fossil fuels, such as coal and oil, for example. They give off plenty of heat when they burn; unfortunately, they give off plenty of other things as well, including the particles that make up smog and soot, the carbon dioxide responsible for the greenhouse effect and the oxides of nitrogen and sulphur that help to make acid rain. A new fuel additive called Carbonex seems drastically to reduce emissions of particles and of nitrogen oxides. It may thus help to solve half the problems.

Tips: If a paragraph lists several problems, the heading should be broad, not focused on just one example.

Why the other options are wrong:

• Greenhouse effect and global warming: This is only one example of the problems mentioned, not the whole focus. This is an example-as-main-idea trap.
• The problems of fire ironed out: This sounds positive and like a conclusion, but this paragraph is describing the problem, not the solution. This is a context shift trap.

Key Vocabulary:

• emissions = gases released
• soot = black pollution

Question Focus: Main idea - source of pollution particles during combustion

Passage Reference: B. To understand the solution, take a closer look at the problem. Fossil fuels are mostly made of carbon, which reacts with oxygen to produce carbon dioxide (in the case of coal) or carbon dioxide and water (in the case of petrol, diesel fuel and other refined oils). The combustion of fossil fuels is never quite complete. Small, unbumt particles of fuel always escape, often as black smoke. These particles contain cancer-causing chemicals and are ever more unpopular.

Why this is correct: This paragraph explains how harmful particles are formed, focusing on incomplete combustion and unburnt fuel. It directly answers the question of where the particles come from.

Tips: Headings with a clear question like "where" often match paragraphs that explain causes or origins.

Why the other options are wrong:

• Problems with fossil fuel: That heading is too general. Here the paragraph explains the mechanism, not the overall problem. This is a partial truth trap.
• Engine power vs. clean emissions: Engine power is never discussed here. This is a wrong section trap.

Key Vocabulary:

• combustion = burning
• unburnt = not burned

Question Focus: Main idea - a solution that creates a new problem

Passage Reference: C. There is a standard fix for this. To reduce the problem of incomplete burning, combustion chambers are routinely flooded with about 25% more air than they need to burn their fuel. The idea is to give the flame more oxygen and hence, increase the efficiency of burning. But there is a snag. Dry air is 21% oxygen and 78% nitrogen. Nitrogen, like carbon, reacts with oxygen at high temperature - in this case producing the nitrogen oxides (NOX) that help cause acid rain. When extra air is added to a combustion chamber, emissions of soot and smog go down but NOX emissions go up.

Why this is correct: The paragraph explains that adding extra air solves one problem but creates another. This clear contrast shows a negative side-effect of a solution.

Tips: Words like "but" and "however" often signal a side-effect or contrast - look for them.

Why the other options are wrong:

• Increase engine efficiency — how it works: Efficiency is mentioned, but the main focus is the new problem created. This is a partial truth trap.
• Problems with fossil fuel: This paragraph is about a solution and its drawback, not the original problem. This is a context shift trap.

Key Vocabulary:

• snag = problem
• emissions = pollution released

Question Focus: Main idea - explanation of Carbonex and its function

Passage Reference: . D. Carbonex, invented by an academic chemist, Dr. David Farrar, at the University of Toronto, and developed by Velino Ventures of Toronto, tries to alleviate this. The active ingredient is a hydrocarbon molecule to which an iron atom is bound. The molecule acts as a carrier for the iron, letting it dissolve in organic liquids like oil and petrol. When sprayed into a flame at the same time as a carbon-based fuel, Carbonex makes it bum more efficiently. The result is fewer particles and less need for extra air.

Why this is correct: This paragraph explains what Carbonex is and how it improves burning efficiency. The focus is clearly on how the additive works.

Tips: When a paragraph explains a process, headings with "how" are often correct.

Why the other options are wrong:

• Iron is the best: Iron is mentioned, but not explained in detail yet. This is an example-as-main-idea trap.
• Carbonex works with more than one fossil fuel: Different fuels are mentioned, but that is not the main point. This is a partial truth trap.

Key Vocabulary:

• efficiently = with less waste
• additive = substance added

Question Focus: Main idea - two stages where Carbonex works

Passage Reference: ir. E. It seems to work at two points during burning. If Carbonex is sprayed into a stream of fuel entering a flame in a combustion chamber, it coats the fuel and deposits iron atoms on the surface of the fuel particles. As the particles enter the outer part of the flame, which is cooler than the core, the more volatile components in the fuel vaporize. The faster these vapours leave the fuel, the sooner oxygen can get to the surface of the fuel particle and react with the carbon. Carbonex appears to speed up the exodus. Fuel particles that survive the hot core of the flame contain several large and unhealthy molecules that do not bum well without a fuel additive. Carbonex also seems to lower the temperature at which these compounds bum, allowing them to disintegrate even while the fuel particle is cooling.

Why this is correct: The paragraph clearly says Carbonex works at two different points in the burning process. The heading matches this structure exactly.

Tips: Numbers like "two" or "three" in the paragraph often match headings with the same idea.

Why the other options are wrong:

• Increase engine efficiency — how it works: That idea is too general. This paragraph focuses on two specific stages. This is a partial truth trap.
• Where the harmful particles come from: Particles are mentioned, but the focus is on how Carbonex acts, not origins. This is a context shift trap.

Key Vocabulary:

• volatile = easily vaporized
• disintegrate = break apart

Question Focus: Main idea - why iron was chosen over other metals

Passage Reference: ling. F. Iron is not the only substance that can manage this trick; any metal should do. Barium, magnesium, manganese, cobalt, nickel and lead have all been tried as fuel additives, but all are toxic and so in disfavour. Iron was picked for Carbonex because it is non-toxic and very effective even in small doses.

Why this is correct: The paragraph compares iron with other metals and explains why iron is preferred. This makes the heading a direct match.

Tips: If a paragraph compares options and gives a reason, choose a heading showing preference or evaluation.

Why the other options are wrong:

• Alternative energy: This paragraph is about fuel additives, not new energy sources. This is a wrong section trap.
• Carbonex works with more than one fossil fuel: Fuel types are not discussed here. This is a context shift trap.

Key Vocabulary:

• toxic = poisonous
• doses = amounts

Question Focus: Main idea - test results across different fuels

Passage Reference: doses. G. Over the past two years, Carbonex has been tested by an independent research group at the Battelle Memorial Institute in Columbus, Ohio. The researchers found that it reduced emissions of particles from diesel engines by 43% and increased the yield of energy from combustion by 1.5-3%. When tested in a light-fuel-oil industrial burner in Geneva, it cut emissions of particles by 67%; added to a coarsely ground bituminous coal it reduced NOX emissions by 25%. In addition to reducing NOX and virtually eliminating black smoke, Carbonex cut the amount of soot left behind in combustion chambers, and so made maintenance cheaper. The fuel additive paid for itself in increased energy efficiency.

Why this is correct: The paragraph gives test results for several different fuels, showing Carbonex works in many situations.

Tips: Lists of examples often support headings about range or variety.

Why the other options are wrong:

• Engine power vs. clean emissions: Engine power is not compared with pollution here. This is a wrong focus trap.
• Carbonex is ideal for factories: That idea is saved for the next paragraph. This is a wrong section trap.

Key Vocabulary:

• yield = amount produced
• maintenance = upkeep

Question Focus: Main idea - best practical use of Carbonex

Passage Reference: ficiency. H. Although Carbonex could be used in petrol for cars, Dr. Farrar thinks petrol is already refined enough to make it unnecessary. The real need for his invention, he thinks, is in plants that bum coal and less refined oils such as furnace and bunker oil. Farewell to those dark

Why this is correct: The paragraph explains where Carbonex is most useful, focusing on industrial plants rather than cars.

Tips: Final paragraphs often focus on applications or future use.

Why the other options are wrong:

• Carbonex works with more than one fossil fuel: Fuel types are not the focus here. This is a context shift trap.
• Alternative energy: This is still about fossil fuels, not alternatives. This is a contradiction trap.

Key Vocabulary:

• refined = purified
• bunker oil = heavy industrial fuel

General Tips:

• Read all headings first and underline key words before matching.
• Focus on the main idea of each paragraph, not one detail.
• Headings follow paragraph order from top to bottom.
• If two headings seem possible, choose the one that covers more of the paragraph.
• Cross out each heading after using it so you do not reuse it.