如图所示，虚线为匀强电场中与场强方向垂直的等间距平行直线，两粒子M、N质量相等，所带电荷的绝对值也相等．现将M、N从虚线上的O点以相同速率射出，两粒子在电场中运动的轨迹分别如右图中两条实线所示．点a、

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如图所示，虚线为匀强电场中与场强方向垂直的等间距平行直线，两粒子M、N质量相等，所带电荷的绝对值也相等．现将M、N从虚线上的O点以相同速率射出，两粒子在电场中运动的轨迹分别如右图中两条实线所示．点a、b、c为实线与虚线的交点．已知O点电势高于c点，若不计重力，则(　　)

A：M带负电荷，N带正电荷

B：N在a点的速度与M在c点的速度大小不相同

C：N在从O点运动至a点的过程中克服电场力做功

D：M在从O点运动至b点的过程中，电场力对它做的功等于零

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D

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During the day we work and play, and at night we sleep. Our bodies rest while we are 1. In the morning we are ready2 again. Our bodies grow 3 while we are asleep. Children who are tired usually 4 sleep. We can get better at our lessons 5 we have had plenty of rest. Boys and girls who are eight or nine years old need ten hours of sleep every night. Our bodies need plenty of 6 when we sleep. If we do not get enough fresh air, we 7 tired when we wake up. While in bed we must not cover our 8. Our lungs(肺) need to get enough fresh air. If we 9 our windows at night, we can have plenty of fresh air. Cool air is better than warm air. Boys and girls who want to be 10 must get plenty of sleep.

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