I can't give my son the leading position. _____, he is too young; secondly, I want

Mr. Smith to have the job. [ ]

A. After all

B. First of all

C. Above all

D. In all

大型工程机械在铁路安全保护区内进行有碍行车安全的施工时应按（）设置专人防护，列车接近前应停止作业。

A.三机一人

B.二机一人

C.一机一人