Mary must keep her mind on her medical studies now, so it would be years ______ she could think about marriage.

A．before

B．when

C．since

D．until

北京和广州两地自转角速度和线速度比较，正确的是（）

A.角速度和线速度都相同

B.角速度相同，线速度北京大于广州

C.角速度相同，线速度北京小于广州

D.两地角速度和线速度都不相同