I don't think that D has some difference with E, since if you "they" have no explicit antedote in answer D, so answer E has the same problem. but B is absolutely not the right answer.
D. Because they desire to be as thin as possible, young female ballet dancers and gymnasts sometimes fail to maintain good eating habits E. Young female dancers and gymnasts sometimes fail to maintain good eating habits because they desire to be as thin as possible
I have found the answer key of the first quetion. It looks like the answer is D. This is not a real GMAT question and this question is from a practicing materal called "GMAT+". However, a lot of people have found errors in the answer key. Hopefully this one is correct.
I actually did a search in yahoo and found a GMAT discussion group. Somebody was discussing the same question and arguing between D and E. A guy said he had the answer key, however he said there were known errors in the answer key. So hopefully the answer for this one is correct. It seems that even the native Americans are confused by the answer E.