研究人员为调查随机选择了一组选民样本。

Researchers selected a random ___sample of voters for the survey.

该研究使用了全国代表性的200所学校样本。

The study used a nationally representative ___sample of 200 schools.

这份报告基于超过50名学生的样本。

This report is based on a ___sample of over 50 students.