Man’s understanding of nature is developing _________. It never stays at the same level.[ ]

A. at the right time  

B. for the first time    

C. from time to time  

D. all the time

关井状态下起下钻作业，只能通过（）坡度的对焊钻杆接头，并采用减压调压阀来控制由于钻具接头过胶芯而引起的波动压力，以延长胶芯使用寿命。