Precipitation occurs when

a. it's warm and moist outside

b. cloud droplets get too heavy and fall

c. water vapor cools and condenses

d. it's dry and cold outside

the cell is the smallest structural and functional unit of living organisms, which can exist on its own. therefore, it is sometimes called the building block of life. some organisms, such as bacteria or yeast, are unicellular—consisting only of a single cell—while others, for instance, mammalians, are multicellular.