Question

which of the following options is a way in which greenhouse gases may naturally be present in the atmosphere?
a respiration
b fossil fuel combustion
c photosynthesis
d car exhaust emissions

Explanation:

• Option A: Respiration is a natural process. All living organisms (including humans, animals) carry out respiration. The chemical equation for aerobic respiration is \(C_{6}H_{12}O_{6}+6O_{2}\to6CO_{2}+6H_{2}O+\text{energy}\). Carbon - dioxide (\(CO_{2}\)) is a greenhouse gas, and this process occurs naturally in the biosphere.
• Option B: Fossil fuel combustion (\(C + O_{2}\to CO_{2}\) for coal combustion, where coal is a fossil fuel) is an anthropogenic (human - made) activity. It is not a natural source of greenhouse gases.
• Option C: Photosynthesis is the process by which plants (and some other organisms) use sunlight, carbon - dioxide (\(CO_{2}\)) and water (\(H_{2}O\)) to produce glucose (\(C_{6}H_{12}O_{6}\)) and oxygen (\(O_{2}\)). The chemical equation is \(6CO_{2}+6H_{2}O\to C_{6}H_{12}O_{6}+6O_{2}\). It consumes \(CO_{2}\) (a greenhouse gas), so it is not a source of greenhouse gases.
• Option D: Car exhaust emissions are due to the burning of fuel (such as gasoline \(C_{8}H_{18}\)) in internal combustion engines. The reactions (e.g., \(2C_{8}H_{18}+25O_{2}\to16CO_{2}+18H_{2}O\)) are human - induced and not natural.

Answer:

A. respiration