ajax下拉列表框做的无刷新省市联动，包括数据库-drop-down list box to do ajax without refreshing the linked cities, including the database

ajax技术，(动态加载列表框,option选项带value值)，例如当你选择某某院系，其班级选项就弹出来-ajax technology (dynamic load list box, option value option with value), such as when you choose certain departments, the class option to pop up