What is the scientific reason for a local condition called a frost hollow?

Correct answer: Cold dense air accumulates in low-lying areas

Direct answer: A, cold dense air accumulates in low-lying areas. During clear and calm nights, the ground can lose heat by radiation and cool the air touching it. Cold air becomes denser than warmer air, so it tends to drain downslope under gravity and collect in valleys, hollows, or other depressions. The accumulated cold air can lower temperatures enough for frost to form, creating a frost hollow. A correctly explains both the movement and the cooling effect. B is wrong because warm air is less dense and generally rises rather than always settling in valleys. C is wrong because valleys can receive sunlight and the absence of sunlight is not the scientific definition; moreover, frost formation does not require zero moisture. D is wrong because oxygen does not become zero in an ordinary frost hollow. Memory cue: cold air is heavy, so it flows downhill and pools in hollows. This is called cold-air drainage or temperature inversion near the surface, and it can increase frost risk.