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

一对齿轮啮合的中心距稍增大后，其传动比不变，但因两齿轮节圆半径随之增大，啮合角会减小。