如何看待OG语法的解释！茫然啊，求教大牛

205. 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

请大家看OG关于DE比较的解释。

Answer to Question 205
The main challenge in this sentence is to observe the agreement of subject and verb (the resulting flow
pattern... is known...) despite the distraction of a complex intervening structure containing several plural
elements (with crests and troughs...). Choices A, B, and D can, therefore, be eliminated because they use an
incorrect plural verb form, are. Choice E uses the correct verb form, is, but it incorrectly introduces a dependent adverbial although clause into a prepositional phrase (with crests ...). Choice D also makes this error. Such
dependent clauses can only occur in the predicates of full clauses. C, the best choice, uses the correct verb form, is, and correctly puts the although clause inside the predicate of the relative clause (that... rapidly).