一个质点经过时间t沿半径为R的圆周运动一周，回到原出发点。在该段时间内质点的平均速度为                     ，平均速率                    。

—The winner of the race is a tall and thin boy with thick glasses.

—Then it _______ be my friend, Mike, who looks like a “meat ball”.

A．mustn’t

B．can’t

C．may　　

D．should