A:doing; done
B:to do; doing
C:to do; done
D:being done; doing
圆柱的底面直径为8,母线长为5,则它的侧面积是( )
某物体自斜面顶端从静止开始匀加速下滑,经过( 2 +1)秒到达斜面中点,从斜面中点到达斜面底端的时间是( )
I don't know ______.