题目

It is illegal for a public official to ask people for gifts or money _________ favors to them. A . in preference to B . in place of C . in agreement with D . in exchange for 答案：D水平放置的金属框架cdef，处于如图所示的匀强磁场中．金属棒ab置于光滑的框架上且接触良好．从某时刻开始abde中的磁通量均匀增加时ab棒中产生由b到a的感应电流，现施加一外力使金属棒ab保持静止，则金属棒ab受到的外力是（　　）A．方向向右，且为恒力B．方向向左，且为恒力C．方向向右，且为变力D．方向向左，且为变力