—What's the weather like today?

—        [ ]

A. It's Tuesday.

B. It's hot.

C.It's a pen.

D.It's on the desk.

如图，一物体恰能在一个斜面体上沿斜面匀速下滑。若沿斜面方向用力向下推此物体，使物体加速下滑，则斜面受地面的摩擦力是（    ）

A．大小为零

B．方向水平向右

C．方向水平向左

D．无法判断大小和方向