_____the people have become masters of their own country that science can reaily serveA only when B it’s only when 选B 为什么

_____the people have become masters of their own country that science can reaily serve
A only when B it’s only when
选B 为什么

如果是选第一个的话,那句子就得倒装了,就应该是
Only when……that can science really serve.

B
这应该是一个强调句型It is ...that...
像这样的，找几个例句，多读几遍，体会一下，以后肯定不会错的。