想要判断集合中是否包含特定元素，应调用哪个方法？

Choose the correct temporal adverb: Newspapers became mainstream ______ the 19th century, while television dominated ______ the mid-20th century.

The phrase 'The court's ruling is legally sound in defining a kind of...' suggests what about the court's decision?

在一项社会调查中，研究者发现某个群体的收入水平与其教育程度呈现正相关关系。以下哪种解释最有可能是导致这一现象的原因？

根据劳动法，劳动者在哪些情况下可以拒绝执行用人单位的管理指令？