单项选择题

大气层又叫大气圈，地球就被这一层很厚的大气层包围着。下列关于大气层的说法中，正确的一项是( )。

A．大气层的厚度大约为700千米
B．云、雾、雨等现象都发生在暖层内
C．大气层的气温随高度增加而降低
D．大气层的空气密度随高度的增加而减小
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