What was the main reason for long periods of rain on early Earth?

Correct answer: Condensation of water vapour in atmosphere

The direct answer is A: long periods of rain occurred mainly because water vapour in the atmosphere cooled and condensed. Early Earth was very hot, and volcanic activity supplied large amounts of water vapour and other gases. As the planet gradually cooled, vapour changed into tiny liquid droplets. These droplets formed clouds; when they became heavy enough, they fell as rain. The reasoning is: cooling atmosphere → condensation of water vapour → clouds and precipitation → repeated or prolonged rainfall. Option A is correct because condensation is the physical process that changes vapour into liquid water and makes rainfall possible. Option B is wrong because early Earth did not have abundant forests, and forests are not the basic cause of planetary rainfall. Option C is wrong because humans did not exist then and cannot create the early atmosphere’s clouds. Option D is wrong because the direction of river flow has no role in producing atmospheric rainfall. A small refinement is useful: condensation caused the rain, while cooling and the large supply of vapour helped make the rainy period long. Memory cue: vapour cools, condenses, clouds form, rain falls.