手信号灯的绿色灯光高举头上左右摇动表示（）信号。

A、出发

B、空线引导

C、有车线引导

D、停车

I just don't understand_____________ that prevents so many Americans from

being as happy as one might expect.

A．why it does

B．what it does

C．what it is

D．why it is