题目内容：
阅读下列短文，回答题。 The sun's heat warms the air and makes it move. This movement is called wind. Different parts of the Earth receive different amounts of heat. Near the equator (赤道) , the sun is overhead and heats the Earth intensely (强烈). Nearer the poles, the sun's rays strike the Earth at a low angle so the heat is not so intense.
A lot of the earth's heat is reflected back into spaces, by the atmosphere, clouds, dust in the air and by water, snow and ice. So some parts of the Earth are warm and some are cold. Warm air tends to rise and creates areas of high pressure. As warm air rises, cold air flows in and replaces it. The greater the pressure difference is, the stronger the wind blows.
A strong wind is usually caused by a difference in__________. A．heat
B．pressure
C．atmosphere
D．air
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