Why can fog collect more in valleys?

Correct answer: Cold dense air can collect in valleys

Direct answer: A, cold dense air can collect in valleys. During calm or clear conditions, air near the ground cools. Cold air is denser than warm air, so it tends to move downward and settle in low-lying places. A valley can therefore act as a natural basin where cool, moist air remains for some time. If this air cools to its dew point, water vapour condenses into tiny droplets suspended near the ground, producing fog. A is correct because it gives both the physical reason and the condition favouring fog. B is wrong: valleys contain air, and still air can actually help fog persist. C is wrong because valleys are not always hot; they can experience strong cooling and temperature inversion. D is wrong because valleys do not destroy water vapour; moisture may condense there. Memory cue: cold air sinks, and fog can gather in low places.