ขอ source code หรือวิธีการทำ list/select menu

<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<script type="text/javascript">
function hideshow()
{
var s1= document.getElementById('bank_name');

if( s1.options[s1.selectedIndex].value=="B1")
{document.getElementById('KTB').style.display = "block";}else{document.getElementById('KTB').style.display = "none";}
if( s1.options[s1.selectedIndex].value=="B2")
{document.getElementById('BBL').style.display = "block";}else{document.getElementById('BBL').style.display = "none";}

}
</script>
          <select name="bank_name" id="bank_name" onChange="hideshow()">
          <option value="XX">กรุณาเลือก</option>
            <option value="B1">ธ.กรุงไทย</option>
            <option value="B2">ธ.กรุงเทพ</option>
                                        </select>

          
             <select name="KTB" id="KTB" style="display:none;" >
            <option value="KTB1">KTB1</option>
            <option value="KTB2">KTB2</option>
</select>
             <select name="BBL" id="BBL" style="display:none;" >
            <option value="BBL1">BBL1</option>
            <option value="BBL2">BBL2</option>
</select>