单项选择题

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 condenses on the particle of salt. The moisture absorbed by the salt particles, dissolves the salt, thus leaving only a slightly salty cloud droplet.
Cloud droplet, formed from condensation of salt particles of dust particles. The reason is that salt particles are usually larger than dust particles. Their larger size is instrumental (有作用的)in making the single rain droplet grow by coalescence with smaller droplets. Since low level clouds don’t rise high enough to produce ice crystals as a source of condensation, it most likely is the salt particles that cause them to become rain clouds.

The passage implies that, in order for rain to fall, it’s necessary for water vapor to （）.

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