题目选项分析

Thunderclouds form when warm, moist air rises into cooler air above, either because of the ground's being warmer than usually or because of the interaction of two air masses, one warm and one cold, forces warm air to rise.

Thunderclouds form when warm, moist air rises into cooler air above, either because the ground is warmer than usual or because the interaction of two air masses, one warm and one cold, forces warm air to rise.