Light aircraft are fitted with carbon monoxide detectors because*1 pointA malfunction of aircraft air conditioners may result in carbon monoxide entering the cabinIf there is a crack in the exhaust, carbon monoxide can enter the cabin when cabin heat is turned onCarbon monoxide will be present if people are smoking on boardThe presence of carbon monoxide indicates that the pilot is flying through poor weather

Light aircraft are fitted with carbon monoxide detectors primarily for the second reason mentioned: If there is a crack in the exhaust, carbon monoxide can enter the cabin when cabin heat is turned on.

This is because the cabin heat in many light aircraft is generated by air that is warmed by being passed over the aircraft's exhaust system. If there is a crack in the exhaust system, carbon monoxide—a colorless, odorless, and poisonous gas—can leak into this air and subsequently into the cabin.

The other options are less likely. A malfunction of aircraft air conditioners may not necessarily result in carbon monoxide entering the cabin. Carbon monoxide is not produced by people smoking onboard. And the presence of carbon monoxide does not indicate that the pilot is flying through poor weather.