empty set

例句：

1. 1. The professor asked the students to fill out an empty set of questions.
   教授要求学生们填写一张空的问题清单。
2. 2.The math teacher had us complete an empty set of problems before moving on to the next lesson.
   数学老师让我们完成一张空的问题清单，然后继续上课。
3. 3. The researcher analyzed the data from the empty set of questionnaires.
   研究员分析了从空问卷集合中收集的数据。