零件表面上的（）引起应力集中使其疲劳强度下降。
A．裂纹
B．气孔
C．缺口
D．粗糙度
参考答案：C
We all know that, _____, the situation will get worse. [ ]
A. not if dealt carefully with
B. if not carefully dealt with
C. if dealt not carefully with
D. not if carefully dealt with
三相交流电源各电动势eA、、eB、eC之间，有（）的关系。
A、eC=eA－eB
B、eA+eB+eC=0
C、eA+eB+eC=0
D、eA=eB+eC
