电力系统的（）是保证电压质量的基本条件。
A.有功功率与有功平衡；
B.频率稳定；
C.无功补偿与无功平衡；
D.供电可靠性。
参考答案：C
His parents never approve of_____ he plans to do, which always annoys him．
A．whatever
B．no matter what
C．no matter when
D．whenever
Only after I sent out scores of job application _______ an offer from a small company.
A．had I receive
B．did I receive
C．I had received
D．I received
