[单项选择]

Surprisingly, there is a relationship between sea salt and the making of rain drops. Most of the water droplets that make up clouds are formed around small, solid particles. In order to fall as rain, the cloud droplets have to increase in size. Clouds from which rain does not fall have droplets that are fairly uniform in size. Clouds that produce rain have some droplets that are much larger than others.
The production of rain depends on the presence of the large droplets that grow in size by coalescence (a fusing together). There is evidence to support the theory that a particle of sea salt serves as the nucleus (核，核心) or center, around which the larger droplets form. Most of the sea salt probably enters the atmosphere from, bubbles that form on the surface of the sea. When the bubbles break, a tiny, even microscopic, droplet of salt water is cast into the air. Once in the atmosphere, the microscopic piece of salt collects water vapor from the air. The water vapor then co
A. dust particles
B. droplets that are uniform in size
C. droplets that are large and small
D. bubbles