如图所示,导体杆ab、cd放在水平放置的金属平行导轨上,匀强磁场竖直向上,导体杆与导轨间的动摩擦因数为,如果使导体杆ab向左做匀速运动,则导体杆cd的运动情况是 ( )
A campus emergency ______ occur at any time of the day or night, weekend, or holiday, with little or no warning.
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