在电阻元件的交流电路中，电流和电压是（）。
A.不同相的；
B.相位成90°角；
C.同相的；
D.相位成某一角度。
参考答案：C
He _______ busy yesterday, or he _______ you with your experiment.
A．was; had helped
B．was; would have helped
C．had been; would have helped
D．were; would have helped
球根花卉按其种植期可分为两类：即（）和（）。
