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?
when cold air moves over the warmer water of a lake, lake - effect snow can be produced.
in the lake effect, warm air, which is lighter than cold air, rises and moves away.
buffalo, new york, gets more snow than most cities in the united states.

Explanation:

The passage first mentions Buffalo, New York is subject to lake - effect snow. Then it explains the process: cold air moves over warmer lake water. Moisture evaporates, warms the air (warm air is less dense and rises). As it cools (moves away from the lake), moisture condenses. If cold enough, it snows. The first option summarizes this main process. The second option only focuses on a part (warm air rising) of the process. The third option is just a fact about Buffalo (a supporting detail, not the main idea).

Answer:

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