当单相桥式整流电路中任一整流二极管发生短路时，输出电压的数值将下降一半，电路变成半波整流电路。
参考答案：错
They'd better ______________ at two. [ ]
A. start
B. to start
C. started
D. starting
Last year, he wrote to tell me that he ______ his new dance. But I ______ of him ever since.
A．practised; didn’t heard
B．has practised; haven’t heard
C．was practising; hadn’t heard
D．was practising; haven’t heard
