Question

1. exhaust plumes are most dangerous when

oa. the surrounding air is calm and cold.
ob. they are visible.
oc. they come from larger towers.

Explanation:

When the surrounding air is calm and cold, exhaust plumes (which contain pollutants) do not disperse well. Calm air means there is little wind to carry the pollutants away, and cold air can create a temperature inversion (a layer of warm air above cold air that traps pollutants close to the ground). Visible exhaust plumes just mean they can be seen (e.g., due to water vapor condensation in some cases) but not necessarily more dangerous. The size of the tower from which they come doesn't directly relate to their danger level (danger is more about pollutant concentration and dispersion).

Answer:

A. the surrounding air is calm and cold.