设总体服从自由度为k的χ^2分布,X1,X2……Xn是取自该总体的一个样本, _ 则 nX=∑(i=1→n)Xi服从χ^2分布,且自由度为()
A campus emergency ______ occur at any time of the day or night, weekend, or holiday, with little or no warning.
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