Which of the following is an example of a fixed collocation in English?

在短视频的人物设定中，以下哪种做法是错误的？

A researcher conducts a survey to understand public opinion on a newly implemented policy. The survey results indicate that 70% of respondents support the policy. However, the researcher later discovers that the survey was primarily distributed through an online platform that is mainly used by a specific demographic group that is known to be more likely to support the policy. What is the most significant limitation of this survey's findings?

在社会主义社会中，敌我矛盾的性质是？

在选择排序中，最坏情况下需要进行多少次交换？