1.
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.（helpful in doing sth&helpful for sb to do sth均可）
(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）
(E) that shoots dead chickens at airplanes has proved helpful in demonstrating
D为什么不对呢？觉得应该是proves itself helpful 啊，再者为什么要用完成时态呢？ | 
