—  I tried many times, but I still failed to complete the experiment.  

— Cheer up. _______ and the problem will be solved. [      ]

A. If have a little effort

B. Having a little effort  

C. If you have a bit effort

D. A bit more effort

绕地球运行的航天站中（　　）

A．可用弹簧秤测物体重力的大小

B．可用天平测物体的质量

C．可用托里拆利实验测舱内的气压

D．以上实验都不可能实现