Anna is a Chinese girl . She is ten . She is in London with her parents. She doesn't know much English. She can't speak English well. Sometimes her friends don't understand her .

    It's Sunday morning .She is going to the zoo to see the panda . She is waiting at a bus stop .At the bus stop she asks a boy how to go to the zoo. But the boy can't understand her .Then she takes out a pen and some paper. She draw a panda on it and shows the picture to the boy .The boy smiles and then shows her the way to the zoo.

在如图所示电路中，电源电动势为E，内阻为r．开关S₁、S₂、S₃、S₄均闭合，C是水平放置的平行板电容器，板间悬浮着一油滴P，下列哪些方法会使P向上运动（   ）