You must follow the directions exactly, and if you should become_______ , you must take the time to go back again and read them over.  

A．confused

B．to be confused

C．to confuse

D．confusing

一物体从长为L的光滑斜面顶端由静止开始下滑，到达底端的速度为v，运动时间为t，则物体的速度为时，下滑的时间为_____；物体到达斜面中点位置的速度为______