Sentence Correction

Source Official guide. Please explain why OA is correct.

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."
(A) crests and troughs that remain stationary
although the air that forms them is moving
rapidly, are
(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