the people have bee 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
网友回答
A. It is only then; that
B. It was that; when
(答案→)C. It is only when; that
D. It was when; then
解析：It is ... that ...是强调句式，强调的是when引导的时间状语从句。句意：只有当人民成为自己国家的主人时，科学才能真正为人民服务