What may cause fog to persist more in a valley during morning?

Correct answer: Accumulation of cold moist air in the valley

Correct answer: B. At night, valley slopes and the ground can cool by radiation. The air in contact with them becomes colder and denser, so it tends to flow downward and collect in the low valley floor. If this air contains enough moisture, cooling can bring it to saturation and produce fog. The surrounding valley sides can limit mixing and sunlight may need time in the morning to warm and disperse the trapped air. B is correct because cold, moist air pooling directly supports persistent valley fog. A is wrong because hot, dry air generally lowers relative humidity and helps clear fog rather than maintain it. C is wrong because water vapour is necessary for condensation. D is wrong because melting rocks has no relevant role in ordinary valley fog. Memory cue: cold air sinks and pools in low places; moist pooled air can remain foggy.