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