题干

静电在各种产业和日常生活中有着重要的应用,如静电除尘、静电复印等,所依据的基本原理几乎都是让带电的物质微粒在电场作用下奔向并吸附到电极上。现有三个粒子a、b、c从P点向下射入由正、负电极产生的电场中,它们的运动轨迹如图所示,则(      )

A:a带负电荷,b带正电荷,c不带电荷

B:a带正电荷,b不带电荷,c带负电荷

C:a带负电荷,b不带电荷,c带正电荷

D:a带正电荷,b带负电荷,c不带电荷

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B

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    One summer, Aunt Betta and Uncle Bill invited me to travel with them back home to Pennsylvania. I had never been1Los Angeles and was eager to meet our relatives.

    Our first night, we drove through the desert, and then through roads cut through hills. The next day we went through the Rocky Mountains. It was2driving up long inclines(斜坡) with abrupt U-shape turns on the steep hillsides, then driving3slowly with great caution. Beyond Colorado, we drove through miles of 4, where I saw actual food being grown. I5some of the crops.

    In one small town we passed through, Aunt Betta decided I needed new6and took me to a country 7. I was used to8dresses and skirts. Now I had my first pair of jeans, a flannel shirt, shorts and a T-shirt. I loved them all.

    9 we reached our destination. Uncle Bill's aunt had a perfect little two-story house. The first thing I 10was a bed in a cute little room — for me! I looked forward to that. It would be like sleeping in a dollhouse. In the backyard, I saw red things11the back fence. Uncle Bill's uncle said, "I think you never ate a 12tomato in L.A." I hadn't. He 13picked one for me.

    I met all my new aunts, uncles and cousins. My cousins taught me how to do cartwheels (恻手翻) in the big front yard. Country people have more14, I thought.

    All the adults cried when we 15, but I took home all the 16of everything I'd seen. Back in L.A., my lungs hurt when I took a deep breath. I 17 the fresh air. I dreamed of homegrown tomatoes. My country clothes seemed out of 18, but I wore them anyway.

    I was two weeks late starting19, but I told my teachers only that I'd been in Pennsylvania and decided to 20all the rest to myself. If only they could have experienced everything that I did!