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下列哪个是磁感应强度的单位(    )

阅读短文，下列问题

      A boy goes to the cinema. He buys a ticket and goes in. But after two or three minutes he comes out. He buys a second ticket and goes in again. After a few minutes he comes out again and buys a third ticket. Two or three minutes after that he comes out the fourth time and asks for another ticket. The girl says to him, “Why are you buying all these? Are you meeting friends in the cinema all the time?”

     “No, I'm not doing that,” the boy says. “But a big woman always stops me at the door and tears(撕掉) my tickets.”