text visibility according to radiobutton list item change using javascript problem

Hi
I have a text box and a radiobuttonlist in one of my forms with 4 radiobutton list items Hide1,Hide2,Hide3 and Display When I select the list item Display the text box should be shown. I tried the below code when the list item Display is at the first in the radiobutton list items it works fine but when I move it to the last its not working 
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
<title>Untitled Page</title>
<script type="text/javascript">
function toggle()
{
var txtObj = document.getElementById('<%=txtBox.ClientID %>');
var rbgObj = document.getElementsByName('<%=rbList.ClientID %>');
for (var i = 0; i < document.getElementsByName('<%=rbList.ClientID %>').length; i++) {
var rbObj = document.getElemen ...