题干
— Talking loudly in a library is impolite.
— In fact, we should also ________ not to cough(咳嗽) or sneeze（打喷嚏）loudly in public.
A：look after
B：be careful
C：take care of
D：look up
答案(点此获取答案解析）
B
，神州飞船在地球表面附近圆形轨道运行周期为
，火星质量与地球质量之比为p，火星半径与地球半径之比为q，则