题目

Thanksgiving Day is on the fourth Thursday of November in the U.S. and on the second Monday in October in Canada.For many Americans, it is the most important holiday apart from（除了）Christmas. Schools, offices and most businesses close for Thanksgiving, and many people make the whole weekend a holiday.Today people celebrate Thanksgiving to remember the early days. The most important part of the celebration is a traditional dinner with foods that come from North America. The meal includesturkey（火鸡）, sweet potatoes and so on.On Thanksgiving there are special TV shows and sports gam如图所示，A、B两点各放一电荷量均为Q的等量异种电荷，有一竖直放置的光滑绝缘细杆在两电荷连线的垂直平分线上，a、b、c是杆上的三点，且ab＝bc＝l，b、c关于两电荷连线对称。质量为m、带正电荷q的小环套在细杆上，自a点由静止释放，则（     ）　　　　　　　　　　　　　　　　A．小环通过b点时速度为              B．小环通过c点时速度为C．小环从b到c速度可能先减小后增大       D．小环做匀加速直线运动