受交变应力的零件发生断裂时的应力，远低于材料的屈服强度。
参考答案：对
Attention, please. All mobile phones must before the meeting starts. [ ]
A. turn
B. be turned off
C. be turning
能疏散风热、疏肝解郁、利咽透疹的药物是
A．桑叶
B．菊花
C．金银花
D．薄荷
