武警战士进行体能训练，双手握住竖立竹竿攀登，以下说法正确的是（ ）
A：他沿竹竿匀速下滑时，所受到的摩擦力方向竖直向上
B：他沿竹竿匀速下滑时，所受到的摩擦力方向竖直向下
C：他沿竹竿匀速上攀时，所受到的摩擦力方向竖直向下
D：他停在空中时增大握杆力，所受到的摩擦力随之增大
—I won't get good grades_______ I study hard.
—That's ______ everyone is trying his best to prepare for the exam.
A. No．It's on the first floor.
B Thank you.
C. Yes，it is．
D. It's next to my classroom.
