The people have become masters of their country. _____ science can really serve the people. [ ]

A. It is only then; that

B. It was that; when

C. It is only when; that

D. It was when; then

某机电工程项目，在防腐管道施工过程中，根据施工工艺安排，在管道焊接时，项目经理部根据施工现场的实际情况，本着尽可能选择优质低耗、劳动强度低的焊接方法，决定采用电弧焊。焊接前明确了焊接技术人员、质检人员、无损探伤人员、焊工及焊接热处理人员的职责，进行统一分工。由于分工合理，工程按期完成，得到业主的好评。

对于受气相腐蚀或腐蚀性较弱的液体介质作用的设备，一般补贴( )玻璃布即可。

A．2～3层

B．3～4层

C．4～5层

D．3～5层