— I don't know .— Because he has to look after his mother.
A:why he is leaving
B:why is he leaving
C:whether he will leave
D:what is he leaving for
甲、乙两颗人造地球卫星,离地面的高度分别为R和2R(R为地球半径),质量分别为m和3m,它们都绕地球做匀速圆周运动,则