I managed to make AJAX Cascading DropDown through this Demo:

http://www.aspsnippets.com/Articles/AJAX-Cascading-DropDown-Example-in-ASPNet.aspx

But when I try to add a submit button which simply call 

protected void Button1_Click(object sender, EventArgs e)
{
   Response.Write(ddlCountries.SelectedValue + "|" + ddlCities.SelectedValue + "|" + ddlStates.SelectedValue);
}

I got 

Invalid postback or callback argument.

Error, how can I get those values?