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当前学科：北京住院医师公共卫生学(综合练习)
题目：
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了解某学校在校生视力减退情况，从全校30个班中，随机抽取6个班，并对这6个班中的所有学生做调查。这种抽样方法称为（）
A . 单纯随机抽样
B . 机械抽样
C . 分层抽样
D . 整群抽样
E . 分层整群抽样
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