— I'm sorry, I ________ attentively.
A:was studying
B:studied
C:had studied
D:have studied
如图所示,在光滑的水平面上有一静止的斜面,斜面光滑,现有一个小球从斜面顶端由静止释放,在小球下滑的过程中,以下说法正确的是( )