牙轮钻头以（）方式破碎岩石。
A、冲击
B、压碎
C、剪切
D、冲击压碎-剪切
参考答案：D
单项式-2a2bm-1与单项式3an-2b是同类项，则m=______，n=______．
_____ I was in the USA, I made a lot of friends there.
A．While
B．Although
C．Unless
D．Until
