材料不同的两物块A和B叠放在水平面上,已知物块A重05kN,物块B重02kN,物块A、B间的摩擦系数f1=025,物块B与地面间的摩擦系数f2=02,拉动B物块所需要的最小力为( )
A campus emergency ______ occur at any time of the day or night, weekend, or holiday, with little or no warning.
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