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
According to United States Air Force officials, a cannon that shoots dead chickens at airplanes has proved helpful in demonstrating what kind of damage can result when jets fly into a flock of large birds. 我觉得楼主和dreadpower的观点是对的, 试试翻译一下, "一种...大炮通过表现出...来证明了它的有用性", 无论如何, demonstrate的动作是cannon发出了的.
如果是这样的话,那不是应该说 a canno ... has been proved to be helpful in doing.. ?