请教大全 sec3 - 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 答案是E

这句话是什么意思?---------怎么也看不明白

另外, 之所以不选A的原因真的只是?----------In A, the logical subject of " to demostrate" probably is " a canon", while " in demostrating" focus the way in which it is helpful (carefield 版主的解释http://forum.chasedream.com/dispbbs.asp?boardID=23&ID=18782)

还有其他理由吗?