在螺纹标注中，应注出螺距的是（）。
A、普通螺纹
B、管螺纹和梯形螺纹
C、细牙普通螺纹、梯形螺纹和锯齿形螺纹
D、所有螺纹
参考答案：C
设全集S有两个子集A，B，若由x∈CSA⇒x∈B，则x∈A是x∈CSB的______条件．
China will put cultural reform and development at ____ important place in____ coming five
years, and ____quite long period of time after that.[ ]
A. an; the; a
B. a; the; the
C. the; a; a
D. the; the; a
