电子记事本类的PDA中可根据用户的要求增加功能。
参考答案:错
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。