Methods: an anonymous questionnaire was completed on a stratified cluster sample of 1735 elementary school and middle school students who were randomly selected from Hegang.
方法：分层随机整群抽取鹤岗市1735名中小学生进行无记名问卷调查。
Methods: an anonymous questionnaire was completed on a stratified cluster sample of 1735 elementary school and middle school students who were randomly selected from Hegang.
方法：分层随机整群抽取鹤岗市1735名中小学生进行无记名问卷调查。
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