_______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

CRTSⅡ型轨道板钢筋骨间的电阻值不应小于（）MΩ。

A、1

B、2

C、3

D、5