下列判断中正确的是（　　）

A．不可能有既省力又省功的机械

B．不可能有省力但不省功的机械

C．可能有做功快但做功少的机械

D．可能有机械效率高但功率小的机械

The car passed through the street ___ are thought was a very dangerous speed.

A.  which

B.  at which

C.  what

D.  at what