根据惠更斯—菲涅耳原理,若已知光在某时刻的阵面为S,则S的前方某点P的光强度决定于波阵面S上所有面积元发出的子波各自传到P点的()
A campus emergency ______ occur at any time of the day or night, weekend, or holiday, with little or no warning.
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