Filtering dropdown content between repeated user controls

Hi,

I have a user control in Update panel. User control has a dropdown with city names. content of my .aspx is structured like this (Updatepanel -> Repeater -> Usercontrol) At runtime in my aspx page I can add as many user controls as I need through repeater control. When a user selects a City in one user control's dropdown, that City should be removed from all the other dropdown's in rest of the User controls. How can I make sure that City selected in one dropdown will not appear in other user controls drop down? I think it can be done through delegates and events... but not sure how to implement it!! Any help would be appreciated...