用所给的动词的适当形式填空。
Look! My sister ______________ (play) with the toy cars.
is playing
--- Can you play tennis with us now?
--- _____. I have to finish my homework first.
A.Yes, I can’t.
B.No, I mustn’t.
C.I’m afraid not.
D.Of course
如图所示,小明用20N的力推着木箱沿着斜面向上做匀速直线运动,木箱的机械能将 (填“增大”、“减小”或“不变”).若小明将推力增大到30N时,则斜面对木箱的摩擦力将 (填“增大”、“减小”或“不变”).