***self 的用法求助

chasekai

according to us 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.
D.that shoots dead chickens at airplanes proves itself helpful to demonstrate
E.that shoots dead chickens at airplanes has proved helpful in demonstrating
答案E，这题倒不难，但是我弄不懂prove itself  helpful与 prove helpful 哪个更好一点，要是遇到类似这样的考点（代词不存在歧义的情况下）应该怎么选择呢？请大家指教，谢谢

liulujun

我认识prove是一个很主观的词汇... cannon不可能主观的去证明自己helpful to do...
弱弱的问一句.. 没记错的话... be helpful to do 和be helpful in doing都可以的吧...

chasekai

我google了下，应该都可以吧。