Why is moisture necessary for precipitation to develop?

The moisture is important for precipitation to develop because it is consisted of water vapor.

The moisture is consisted of water vapor. The water vapor is less dense than the air and it moves upwards. When it reaches certain height where the temperatures drop to a point where the water vapor transforms from gas to liquid, the process of condensation, clouds start to form. The clouds when saturated enough with microscopic drops of water start to produce rain, thus precipitation. 

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Your answer is d. silli hope this ! : ) i left an explanation down below if you need it: a sill forms when hot magma squeezes between layers of rock to form a hardened sheet of rock. a sill is basically a tabular sheet intrusion between old layers of sedimentary rock, volcanic beds, or in the direction of foliation inherent in metamorphic rock. the word sill means a concordant intrusive sheet.