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 toughs that remain stationary although the air that forms them is moving rapidly, are known as "standing waves."

A) crests and toughs that remain stationary although the air that forms them is moving rapidly, are
B) crests and toughs that remain stationary although they are formed by rapidly moving air, are
C)crests and toughs that remain stationary although the air that forms them is moving rapidly, is
D)stationary crests and toughs although the air that forms them is moving rapidly, are
E)stationary crests and toughs although they are formed by rapidly moving air, is.

completly lost on the OG explanation.Can anybody throw some light in detail?