— I was told that an accident happened some day last month. Could you tell me where and when exactly?

— I'm sorry I can't remember when ______ accident happened. I only remember it was _____ Saturday

     morning, when it was snowing.[ ]

A. the, the

B. the, a

C. a, a

D. a, the

有两个行星A和B（A和B之间的相互作用不计），它们各有一颗靠近其表面的卫星，若这两颗卫星的周期相等，由此可知[ ]

A．行星A、B表面重力加速度之比等于它们的半径之比

B．两颗卫星的线速度一定相等

C．行星A、B的质量一定相等

D．行星A、B的密度一定相等