—  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

进行碳弧气刨操作时电弧切割时烟尘大，操作者应佩戴（）。