It takes forever to learn the rules and        you've learned them they can be used freely.[ ]

A. unless

B. before

C. though

D. once

弹簧管压力表中的（）一端焊接空心的表接头，连杆机构和指针组成。

A.圆形空心弹簧管

B.环形空心弹簧管

C.扁曲空心弹簧管

D.都不对