[简答题]Read the following paragraph and write the topic sentence.     Tornados form under very special weather conditions, and these special weather conditions occur most often in inland areas, such as the central United States. A tornado forms when a layer of warm, dry air is on top of a layer of cooler, moist air. This combination of dry, warm air above wet, cool air creates a condition that causes the lower layer of air to lift up. As the lower air rises, both layers of air begin to rotate, to turn around and around. The air begins to rotate faster and faster because of centrifugal(离心的) force. The tornado has a center called an “eye” and the air rotates quickly around this eye.