Question

creating lake-effect snow
situated on lake erie, the city of buffalo, new york, is one of the snowiest cities in the united states. like other cities on large lakes, it is subject to a phenomenon called lake - effect snow. water retains heat, so lake water is often warmer than the air. in the lake effect, cold air moves over warmer lake water, initiating a process that can produce large snowstorms. moisture from the lake evaporates into the cold air, warming it. because warm air is less dense than cold air, it begins to rise. as it rises and moves away from the lake, it cools down. as the air cools, moisture condenses and forms into clouds. if the temperature is cold enough, the moisture falls to the ground as snow.
what is the main, or central, idea of the passage?
in the lake effect, warm air, which is lighter than cold air, rises and moves away.
when cold air moves over the warmer water of a lake, lake - effect snow can be produced.
buffalo, new york, gets more snow than most cities in the united states.

Explanation:

The passage starts by introducing Buffalo (on Lake Erie) as a snowy city due to lake - effect snow. Then it explains the process: cold air over warm lake water (evaporation, warm air rising, cooling, moisture condensing as snow). The first option only focuses on warm air rising. The third option is just about Buffalo's snow (a detail). The second option captures the main process (cold air over lake water leading to lake - effect snow) which is the central idea.

Answer:

When cold air moves over the warmer water of a lake, lake - effect snow can be produced.