Why can fog collect more in valleys during morning?

Correct answer: Cold dense moist air can remain in valleys

Direct answer: C. At night, air near the ground can cool. Cooler air is denser than warmer air and may flow downslope or settle into a depression such as a valley. If this air contains moisture, its temperature can fall to the dew point. Water vapour then condenses into tiny droplets suspended in the valley, producing morning fog. A is wrong because valleys are not always hot; they can collect cold air. B is wrong because valleys contain air, although their shape can restrict mixing and drainage. D is wrong because a valley does not destroy water vapour; cooling changes some vapour into droplets. The effect is often strongest near dawn, before solar heating disperses the fog. Memory cue: valleys are low places where cold, moist air can pool.