普通三螺杆泵必须按既定的方向转动，否则将导致（）
A.径向力无法平衡
B.轴向力无法平衡
C.圆周力无法平衡
D.原动机过载
参考答案：B
A．(A) The trees are being cut down.
B．(B) The trees are blocking my view.
C．(C) The trees grow better around the office.
D．(D) The trees help cool my office.
怎样预防肺结核?
