Populate one dropdown list based on the selection of other dropdown list

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stored-procedure

i have a dropdown list campus as head office,campus1,campus2

this is campus load method:
private void LoadDDCampus(int intEmployeeID)
{

DAOEmployeAttendence objDAOEmployeAttendence = new DAOEmployeAttendence();
DataTable objDataTable = new DataTable();
objDataTable = objDAOEmployeAttendence.spGetInfoAboutUser(intEmployeeID);
ddBranch.DataSource = objDataTable;
ddBranch.DataTextField = Convert.ToString(objDataTable.Columns["CampusName"]);
ddBranch.DataValueField = Convert.ToString(objDataTable.Columns["CampusID"]);
ddBranch.DataBind();

ViewState["CampusID"] = Convert.ToString(objCrypto.Decrypt(Server.HtmlEncode(Request.Cookies["UserSessionCookie"].Values["CampusID"])));

}
when i select head office or campus1 or campus 2 ,suppose i select head office then in another dropddown list should only show that head office empolyees in recieved by dropdown list. in other words if i selcet any campus then in dropdown of received by only shows that specific users according to campus selection in dropdown of campus.
div class="control-group">
<label for="textarea" class="control-label">
Received By</label>

its method load dd is:
public void loadReceivedBy()
{
DataTable dt = new DataTable();
dt = provider.spReceivedby();
ddReceivedby.DataSource = dt;
ddReceivedby.DataTextField = Convert.ToString(dt.Columns["FullName"]);
ddReceivedby.DataValueField = Convert.ToString(dt.Columns["EmployeeID"]);
ddReceivedby.DataBind();
}
and its store procedure is spReceivedby():
ALTER PROCEDURE [dbo].[spReceivedby]
AS
SELECT '0' AS EmployeeID, 'Select' AS FullName
UNION
SELECT EmployeeID,FullName
FROM EmployeeProfile
RETURN
so what should i do??