拱坝破坏的主要形式有哪两种？
参考答案：⑴坝肩岩体发生滑动破坏，为主要形式；
⑵坝肩岩体变形量过大，引起坝体的不利应力。
Some animals, in order to keep themselves________by the enemies, go out for food at night. [ ]
A. not to be attacked
B. against being attacked
C. from being attacked
D. not being attacked
不能通过单质直接化合得到的是
A、FeCl3
B、FeS
C、CuS
D、NaCl
