Meaning: 'Polluting' refers to the act of causing pollution, particularly in relation to vehicles and their emissions.

Wrong options:

• introduced: 'Introduced' is a verb that does not fit grammatically as it cannot describe the state of vehicles in this context.
• efficient: 'Efficient' is an adjective that describes performance rather than pollution, making it contextually inappropriate for this blank.
• slashed: 'Slashed' is a verb that indicates a reduction but does not describe the vehicles themselves, thus it does not fit here.
• modern: 'Modern' is an adjective that describes something contemporary but does not relate to pollution levels, making it irrelevant in this context.
• expensive: 'Expensive' is an adjective that refers to cost, which does not relate to the environmental impact of vehicles.

Correct answer: efficient

Explanation: The term 'efficient' is an adjective that describes the engines in terms of their performance and fuel usage. It fits grammatically as it modifies 'engines' and is contextually relevant to the discussion of improvements in vehicle technology.

Meaning: 'Efficient' refers to the ability to achieve maximum productivity with minimum wasted effort or expense, particularly in relation to fuel consumption.

Wrong options:

• introduced: 'Introduced' is a verb that does not fit grammatically as it cannot describe the engines in this context.
• polluting: 'Polluting' is an adjective that describes environmental impact, which does not relate to the performance of engines.
• slashed: 'Slashed' is a verb that indicates a reduction and does not describe the engines, making it contextually inappropriate.
• modern: 'Modern' is an adjective that describes something contemporary but does not specifically address the performance of engines, making it less relevant here.
• expensive: 'Expensive' is an adjective that refers to cost, which does not relate to the efficiency of engines.

Correct answer: slashed

Explanation: The word 'slashed' is a verb that indicates a significant reduction in emissions. It fits grammatically as it describes the action taken regarding pollutant levels and aligns contextually with the discussion of emissions reduction since 1970.

Meaning: 'Slashed' means to cut or reduce something drastically, in this case, the emissions of pollutants.

Wrong options:

• introduced: 'Introduced' is a verb that does not fit in this context as it does not relate to the reduction of emissions.
• polluting: 'Polluting' is an adjective that describes the nature of vehicles but does not indicate any action regarding emissions, making it irrelevant here.
• efficient: 'Efficient' is an adjective that describes performance rather than an action taken on emissions, thus it does not fit.
• modern: 'Modern' is an adjective that describes something contemporary and does not relate to the action of reducing emissions.
• expensive: 'Expensive' is an adjective that refers to cost and does not relate to the reduction of emissions.