教我一下这道题---**题型特征** **Question Stem:** 如图所示，电阻不计的光滑足够长“U”形金属框架固定在水平面上，空间存在竖直向下的匀强磁场。电阻为R的ab棒在恒力F作用下从静止开始向右运动，则（ ） **Options:** A. ab棒做匀加速运动 B. 感应电动势先增大后减小 C. 回路中电流先增大后不变 D. ab棒受到水平向右的安培力 **Diagram Description:** * **Type:** Schematic diagram illustrating a physical setup. * **Main Elements:** * A U-shaped horizontal metal frame is shown. The two parallel arms are connected by a movable rod labeled "ab". * The rod "ab" is positioned on the U-shaped frame, perpendicular to the arms. * An arrow labeled "F" points horizontally to the right, indicating an applied force on the rod "ab". * Several downward arrows labeled "B" represent a uniform magnetic field perpendicular to the horizontal plane. The magnetic field is directed vertically downwards. * Labels "a" and "b" mark the ends of the rod. **Other Relevant Text:** 一根棒，有一个恒定的外力作为动力，拉着棒运动