Why the other options are wrong:

• thickening substance: Thickening substance is not the property used in the comparison. The passage compares heating power.
• liquefied elements: The report does not compare liquefied elements in the two gases; it compares their heating power.
• cooling down power: Cooling down power reverses the kind of effect being described. The stated comparison is about heating.

Correct Answer: chimneys

Why this is correct: The study explicitly compares trees to chimneys because soil methane travels through their roots, stems and leaves into the atmosphere.

Why the other options are wrong:

• tunnels: Tunnels is not the comparison used in the report; it names chimneys.
• channels: Channels can describe transport in general, but the question asks for the report's named comparison, chimneys.
• roots: Roots are part of the route methane travels through; the trees themselves are compared to chimneys.

Correct Answer: higher than expected.

Why this is correct: The report says methane fluxes are higher than expected in wet tropical regions. Transport through trees could help explain the observation.

Why the other options are wrong:

• lower than expected.: Lower reverses the reported direction: the fluxes are higher than expected.
• exactly as expected.: The fluxes exceed expectations rather than matching them exactly.
• never measured.: The report describes fluxes and a possible explanation; it does not say they have never been measured.

Correct Answer: a photolytic reaction.

Why this is correct: Rice says light at a certain intensity and wavelength could create a photolytic reaction that produces methane. He leaves this possibility open rather than declaring it proven.

Why the other options are wrong:

• direct release of soil methane by roots.: Soil methane moving through roots is the other mechanism discussed, not the light-induced reaction.
• loss of all leaf chlorophyll.: Loss of chlorophyll is not identified as a methane-producing mechanism.
• digestion by ruminant animals.: Ruminants are earlier examples of methane sources, not the reaction caused by particular light conditions.

Correct Answer: whether plants produce methane or transport it from soil.

Why this is correct: The report contrasts the proposal that plants produce methane with evidence that trees transport methane made in soil. Rice also leaves open possible methane production under particular light conditions.

Why the other options are wrong:

• whether methane heats air as strongly as carbon dioxide.: Heating power is background information, not the central question about plants' role in methane emissions.
• whether plants can grow without sunlight.: Light is discussed as a possible trigger for methane production, not as a requirement for plant growth.
• whether cows emit less methane than trees.: Cows are named among methane sources, but their emissions are not compared with trees' emissions.