如图所示的电路,电源电压为6V,当开关S闭合时,灯L1、L2都不亮,电压表示数约为6V,则产生故障的原因可能是 _________ .
灯L1灯丝断了,或接触不良.
Mary went to the box office at lunch time, but all the tickets ____ out. [ ]
A. would sell
B. had sold
C. have sold
D. was selling
聚合物溶液纺丝