885SEC3，23

kingdoms

这道题在白勇语法中有详解, 其中也没有说helpful to do sth 不能用, 而只是强调doing 和that从句修饰名词得区别. 在白勇语法103页, 大家可以看一看.另外对prove的用法讲的很详细.

championqian

For 23;

There exist a comparison in this sentence. Cannon is used to demonstrate what kind of damage can result when jets fly into a flock of large birds.

The comparison is between damages caused by "jets (that) fly into a flock of large birds" and "a cannon (that) shoots dead chickens at airplanes". If the later part of this sentence is changed to "when jets flying into a flock of large birds", then we can choose "a cannon shooting dead chickens at airplanes". Both stress action!