一匀强电场的电场强度E随时间t变化的图象如图所示，在该匀强电场中，有一个带电粒子在t=0时刻由静止释放，若带电粒子只受电场力作用，则下列说法中正确的是（ ）
A．带电粒子只向一个方向运动
B．0～2s内，电场力做正功
C．2s末带电粒子回到原出发点
D．4s末带电粒子回到原出发点
带电粒子在t=0时刻由静止释放，在0-1s内带电粒子沿着电场力方向做匀加速直线运动，
1-2s内沿原方向做匀减速直线运动，2s末速度减为零；
2-3s内沿反方向做初速度为零的匀加速直线运动，
3-4s内继续做匀减速直线运动，4s末速度减为零，回到出发点．
A、由上分析可知，带电粒子做往复运动．故A错误．
B、0～1s内电场力做正功，1～2s内电场力做负功．故B错误．
C、D，2s末带电粒子离原出发点最远，4s末带电粒子回到原出发点．故C错误，D正确．
故选D
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| Two friends went to a city and 1 in a big hotel. Their room was on the 56th floor, and there was a fast lift, so they went up and down very 2 . They came back to the hotel very late one Saturday 3 ,and the liftman said to them," The electricity (电) stopped today. You can't 4 the lift now, " The two friends said. "We 5 to go upstairs (上楼) to our room." They left their coats downstairs because they were 6 , and began to climb. It took a 7 time, but they sang some interesting songs. At 12:00, they 8 the 56th floor. Then one of them put his hand in his pocket (口袋) and stopped. He said to 9 , "Now. I'm going to sing the most interesting song of all. Its name is: I 10 the key to the room downstairs in my coat." | ||||
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