He is not easy-going, but his trust in his friends will last forever once _______.

A．to be gained

B．is gained

C．being gained

D．it is gained

施工现场对人员的控制，坚持作业人员（），特别是重要技术工种、特殊工种、高空作业等，做到有资质者上岗。

A.培训上岗

B.持证上岗

C.三级教育

D.安全上岗