已知一年级三个班的人数及数学平均数分别为:1班20人,平均分数为60:2班30人,平均分数为65: 3班40人,平均分数为70分,则全年级平均分为( )
A campus emergency ______ occur at any time of the day or night, weekend, or holiday, with little or no warning.
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