ASP.NET Load Data From Database to Multiple Dropdown Lists

I have a database of cities and zip codes. The webpage is for a delivery service. To make a delivery, I need to know the pickup city and zip code and the dropoff city and zip code. The webpage will have 4 dropdown lists. The user will select the pickup city from dropdown 1 which will populate zip codes dropdown 2. The user will also select the dropoff city from dropdown 3 which will populate the zip codes dropdown 4. I need to retain the selections from all dropdowns after the selections are made. The user then hits the submit button which passes the selections to page 2 for further processing.