在潮湿的环境下，正常状态时，交流特低电压应选（）。
A、12V
B、24V
C、36V
D、42V
参考答案：A
一元二次方程（2x+1）（x-3）=0的根是______．
Tom was about to close the window _____ his attention was caught by a bird. [ ]
A. when
B. if
C. and
D. till
