前人已经讨论过这个题了，
http://forum.chasedream.com/dispbbs.asp?boardID=23&ID=16347
http://forum.chasedream.com/dispbbs.asp?boardID=23&ID=18782
可是他们没有说为什么Ｄ错了。我觉得D挺好的呀。
现在时态，然后还help to do sth.
前人讨论 885-sec2-13的时候，还提过help的用法呢。
转：
use...to do sth, help (to ) do sth, (help 的用法见OG179, 216, 236)
23. According to United States Air Force officials, a cannon shooting dead chickens at airplanes has proved helpful to demonstrate what kind of damage can result when jets fly into a flock of large birds.
(A) shooting dead chickens at airplanes has proved helpful to demonstrate
(B) shooting dead chickens at airplanes has proved itself helpful as a demonstration of
(C) shooting dead chickens at airplanes proves itself helpful as demonstrating
(D) that shoots dead chickens at airplanes proves itself helpful to demonstrate
(E) that shoots dead chickens at airplanes has proved helpful in demonstrating
be helpful in doing是固定搭配；itself指代cannon，逻辑错；
茅塞顿开。谢之！
还小声地问一句，这个题是否也适合牛牛们说的，如果没有明显的时间状语，不要轻易改变原句的时态.
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