先将总体各个观察单位按照某一标志顺序排列编号并分成数量相等的组,使组数与取样数相同。然后从每组中依事先规定的机械次序抽取对象。这是( )
A campus emergency ______ occur at any time of the day or night, weekend, or holiday, with little or no warning.
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