如图所示，细线拴一带负电的小球在竖直向下的匀强电场中运动。假

如图所示，细线拴一带负电的小球在竖直向下的匀强电场中运动。假设小球所受的电场力qE与重力G的大小关系为qE=G，且小球运动到最低点a处的速率为v_a，运动到最高点b处的速率为v_b，则v_a与v_b的关系为（）

A．v_a>v_b	B．v_a=v_b
C．v_a<v_b	D．条件不足，无法确定

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