[问答题] 简述活塞的功用

旅游心理学研究的人主要包括（） ["旅游者","潜在旅游者","服务人员","管理人员","领导人员"] You use Microsoft .NET Framework 4 to create a Windows Presentation Foundation (WPF) application.  You write the following code segment. (Line numbers are included for reference only.) The TextBox control is data-bound to an instance of the Contact class.   You need to ensure that the Contact class contains a business rule to ensure that the ContactName property is not empty or NULL. You also need to ensure that the TextBox control validates the input data.  Which two actions should you perform?（） ["","","","","",""] 下面哪种药物不属于大环内酯类抗生素（） ["红霉素","土霉素","罗红霉素","替米考星"] 按照现代冲突观来看，冲突的作用有积极性的，建设性作用和（）的破坏作用。 ["目的性","明确性","结果","消极性"] 人的需要最高级的层次是（）。 ["安全需要","社交需要","尊重需要","自我实现需要"] 简述活塞的功用