标题: 885 section3-23? [打印本页] 作者: xiaoyue45084508 时间: 2004-1-12 11:59 标题: 885 section3-23? 885-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 请问D，E选项怎么区分？时态为什么用现在完成时？ helpful(adj)的用法只在词典里找到以下2种： helpful+sth helpful to sb 而为什么用in demonstrating呢？ 谢谢指点！
