The peaks of a mountain range, acting like rocks in a streambed, produce ripples in the air flowing over them; the resulting flow pattern, with crests and troughs that remain stationary although the air that forms them is moving rapidly, are known as “standing waves.” B crests and troughs that remain stationary although they are formed by rapidly moving air, are C crests and troughs that remain stationary although the air that forms them is moving rapidly, is D stationary crests and troughs although the air that forms them is moving rapidly, are E stationary crests and troughs although they are formed by rapidly moving air, is E为什么不对呢，看这也可以阿 忘记贴正确答案了，答案是c | 
