如下图,大长方形被分为四个较小长方形,已知四个长方形的面积已标示出来,且这个大长方形的长和宽均为整数,那么图中双向箭头之间的部分是多长?( )
A campus emergency ______ occur at any time of the day or night, weekend, or holiday, with little or no warning.
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