题目

London has a population of over 7,000,000. From about the year 1800 until World War Ⅱ, London was the biggest city in the world. But now there are many cities which are much bigger than it.  London is famous for many things. Tourists come from all over the world to visit its famous buildings and hear the famous clock, Big Ben.  Like many big cities, London has Problems with traffic and pollution which stop the city from developing. Over one million(百万) people a day use the London underground, but there are still too many cars on the streets. The air isn’t clear, but it is any如图为一电源电动势为E，内阻为r的恒定电路，电压表A的内阻为10kΩ，B为静电计，C1、C2分别是两个电容器，将开关闭合一段时间，下列说法正确的是（　　）A．若C1＞C2，则电压表两端的电势差大于静电计两端的电势差B．若将变阻器滑动触头P向右滑动，则电容器C2上带电量增大C．C1上带电量为零D．再将电键S打开，然后使电容器C2两极板间距离增大，则静电计张角也增大