某市有小学200所,要在该市小学生中抽取一个容量为80的随机样本,考虑到小学生年龄偏低,允许一所小学抽取2个学生,应采用( )
A campus emergency ______ occur at any time of the day or night, weekend, or holiday, with little or no warning.
免费的网站请分享给朋友吧