一金属容器置于绝缘板上，带电小球用绝缘细线悬挂于容器中，容器内的电场线分布如图所示．容器内表面为等势面，A、B为容器内表面上的两点，下列说法正确的是（　　）

       Do you still get free plastic bags from the supermarkets? Things have changed.
       China has banned（禁止）free plastic bags in  shops and supermarkets，and people have to pay for using plastic bags. The rule started on June 1st. It came because our country tried to make less litter. Making super-thin plastic bags has also been banned.
       The Chinese once used about 3,000,000,000 plastic shopping bags a day，and they have caused pollution of the environment. The bags have become a main cause of plastic pollution because they are easy to break and people throw them away here and there. So the Chinese people are encouraged to bring their own bags for shop ping.
       What kind of shopping bag is the best to bring? Some students in Chongqing have a good idea. They make their own shopping bags. They use old clothes to make cloth bags，and send them to their parents as presents. They also ask their parents and friends to use cloth bags instead of plastic ones. They think it is their duty to protect the environment.
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