how to view only the list of the selected iterm in a dropdownlist

Add ECD dropdown values in ispostback,then try below code,  

<pre lang="c#">protected void ECDdd_SelectedIndexChanged(object sender, EventArgs e)
    {
try
{

if (ds.Tables[0].Rows.Count > 0)
            {
                ECDschooldd.Items.Clear();
                for (int i = 0; i < ds.Tables[0].Rows.Count; i++)
                {
                    
                     ECDschooldd.Items.Add(ds.Tables[0].Rows[i].ItemArray[0].ToString());
  }


}                

  }