Why is the joining of cloud droplets useful for rain?

Correct answer: Because larger drops can become heavy and fall

Direct answer: D, because larger drops can become heavy and fall. Cloud droplets are initially extremely small and light, so air currents can keep them suspended. When droplets collide and join, the resulting drop has greater mass and size. As it grows, gravity acts more strongly relative to the upward support of air, and the drop may fall from the cloud as rain. In some clouds, ice particles also grow and collect water before falling, but the basic idea remains growth into particles large enough to descend. A is wrong because joining does not make droplets disappear; it combines them. B is unrelated because no fire is involved. C is wrong because droplet coalescence does not stop air movement. D correctly connects coalescence, increased drop size, gravity and rainfall. Memory cue: tiny droplets float; joined, heavier droplets fall.