Rainfall is one of the primary elements in climate and a factor of tremendous importance in the distribution of plant and animal life. Clouds contain huge numbers of tiny drops of moisture. Raindrops are formed when these tiny drops are enlarged, first by moisture from the surrounding air condensing on them and then by combining with other drops during their falling down. Raindrops vary in size from about 0.02 inches to as much as 0.33 inches in thunderstorms. From the time they leave the bottom of the cloud, they change into vapor and, if the cloud is high, the air warm and dry, and the raindrops small, so that they fall slowly, they may become vapor completely before they reach the earth. There are thousands of stations throughout the world where rainfall observations and records are made. Included in such records is the fall of snow, reduced to its equivalent in rain. Rainfall is measured in terms of (按照……) inches, by means of a simple instrument or by more complex electrical or weighing devices placed where eddies (逆流) do not get in the way of the normal fail of the raindrops. In addition to the daily, monthly, and annual totals, the depth of individual rainfalls and their intensity and other related facts are recorded. Raindrops become the largest when ______.
A. they leave the cloud B. they fall slowly C. there is a thunderstorm D. there is warm air