阅读下面短文，从每小题所给的A、B、C、D四个选项中选出最佳选项。
A
A taxi company in one of Japan's big cities is introducing silent taxi drivers．Miyako Taxi，which is based in Kyoto，showed the new idea in March．The business is recently operating five "Silent Taxis" across the city．
The cars look like any ordinary taxi，but a notice written on the back of the passenger seat tells passengers that they have entered one of the new silent cars．Drivers will offer a greeting when their passenger enters the car，and after making sure the route(路线)they will not start small talks．The drivers are only allowed to take their part in a conversation，only if they are spoken to first．Of course，they are also allowed to communicate in emergency(紧急的) situations．
While introducing the idea to the public，a spokesman from Miyako Taxi said，"This service is currently in a trial(试验)period．It is introduced in order to create an in-car environment that provides the most comfortable ride for our passengers through reducing the driver's speaking．"
According to reporters from Japan Today，an online newspaper based in Tokyo，the taxi company made the decision after hearing reports from passengers that they felt bored to be forced to chat with their drivers．
At the end of the trial period，the company will see how popular the silent taxis are with passengers before deciding whether to roll it out across more of their drivers or not．
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是三个完全相同的小灯泡，串联后接在电压为6V的电路中，原来三个小灯泡都正常发光，现在三个小灯泡都不亮了．用电压表接在a、c两端时，示数是6V；接在a、b两端时，示数是0．接在b、d两端时，示数也是0，那么灯丝断了的小灯泡是（ ）