有一组正序对称向量，彼此间相位角是120°，它按（）方向旋转。
A.顺时针旋转
B.逆时针旋转
C.平行方向旋转
D.螺旋向旋转
参考答案：A
To lose themselves in the net bar _____ on teenagers' future life.[ ]
A. have a bad effect
B. has a bad effect
C. have a good effort
D. has a good effect
如图所示，水平桌面上叠放着A、B两物体均与桌面相对静止不动，则B物体受力个数为：
A．3个 B．4个 C．5个 D．2个
