六一儿童节那天,全班45人到颐和园去玩,有33人划了船,20人爬了山,5名同学因身体不好,他们既没划船也没爬山,他们游览了长廊。问:既划了船也爬了山的同学有多少?( )
A campus emergency ______ occur at any time of the day or night, weekend, or holiday, with little or no warning.
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