A couple "if" statements would do the trick.
Then at the top of your page, first define all the record sets that are possible.
Then, wrap the entire set of record sets in an if statement that checks to see if submit was pressed.
Then, you will need to wrap EACH recordset into another if statement which checks if the value of the dropdown is equal to what ever you want to test against. If it is equal, then it executes that particular recorset.