Why are cloud and fog considered different despite structural similarity?

Correct answer: The main difference is height or location

Answer: C. Both clouds and fog are made mainly of tiny suspended water droplets or, in sufficiently cold conditions, ice particles. Their essential structural material is therefore similar. The practical difference is where the visible condensed moisture occurs. A cloud is suspended above the ground in the atmosphere, whereas fog is a cloud-like layer at or very near the ground and can reduce visibility. The reasoning is: water vapour condenses into tiny droplets; if the droplets remain aloft, we call the formation a cloud; if it touches or lies close to the surface, we call it fog. A is wrong because fog contains water droplets. B is wrong because clouds are not rocks. D is wrong because both commonly contain droplets. A useful warning is that “fog is a cloud at the surface” is a helpful simplification, but location is the key classification clue. Memory cue: same condensed water, different position—cloud above, fog near the floor.