公制三角螺纹之螺牙角度为（）
A.30°
B.45°
C.60°
D.75°
参考答案：C
girls took part in the Happy Girl Competition but only few of them succeeded.
A．Million of
B．Many million of
C．One million of
D．Millions of
液压传动系统由液压动力元件（）液压辅助元件和工作液体组成的。
A.液压执行元件
B.液压密封元件
C.液压控制元件
