A, B, and D are clearly wrong because of the "are" at the end. However, the last two choices are similar and both appear to be correct in terms of verb tense and pronoun agreement, but E is wrong and C is the correct answer.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
Why is C more correct than E? Because of the word although. Although signals a specific linguistic element - mainly that something occurred even though something that appears contrary to it is evident. For simplicity's sake, let's interchange although for the phrase "even though" because even though is more often used in spoken English:
"Harry Potter is a great wizard even though he is a young boy" (I just watched Harry Potter and the Order of the Phoenix =P).
Notice how the thing that is being contrasted (great wizard) occurs just before the even though in the sentence. Now let's change even though back to although:
"Harry Potter is a great wizard although he is a young boy"
Let's look at choice C:
(C) crests and troughs that remain stationary although the air that forms them is moving rapidly, is
What is being contrasted? The "stationary-ness" of the crests and troughs.
--> Harry Potter is a great wizard although
--> crests and troughs that remain stationary although
Now let's look at E:
(E) stationary crests and troughs although they are formed by rapidly moving air, is
Notice how what is being contrasted is far away from the although. It is actually a little bit unclear what is being contrasted.
Convert this to the Harry Potter analogy:
"The great wizard Harry potter although he is a boy"
Hmmm....this is far from clear. In fact, the use of the although in these last two sentences suggests something is coming up to clarify the contrast:
"stationary crests and troughs although they are formed by rapidly moving air, are slow and dormant"
"The great wizard Harry potter although he is a boy, is very tall."
Lesson: Make sure what is being compared/contrasted by "although" is clear and close to the word although*
*unless although is used at the beginning of the sentence ==> "Although Harry Potter is a young boy, he is a great wizard"