大肠杆菌A菌株(met-bio-thr+leu+)和B菌株(met+bio+thr-leu-)在U型管实验培养后出现了野生型(met+bio+thr+leu+),证明了这种野生型的出现可能属于( )。
A campus emergency ______ occur at any time of the day or night, weekend, or holiday, with little or no warning.
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