script与FORM的问题

<html>
<head>
<title></title>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
<script language="JavaScript1.2">
function HiddenShow(obj)
{
var returnValue;
for(iIndex=0;iIndex<obj.length;iIndex++)
{
if(obj.options[iIndex].selected)
{
returnValue=obj.options[iIndex].value;
}
}
if(returnValue=="B")
{
//TextIsHidden.style.visibility="hidden";
//TextIsHidden.disabled=true;
                this.form1.t1.style.visibility="hidden";
}
else
{
//TextIsHidden.style.visibility="";
//TextIsHidden.disabled=false;
                this.form1.t1.style.visibility="";
}
}
</script></head><body bgcolor="#FFFFFF" text="#FFFFFF"   background="../picture/wave1024.jpg" onload="">
<form name="form1" method="post" action="">
  <select id=SelectAORB onchange="HiddenShow(this)">
    <option value="A">A</option>
    <option value="B">B</option>
  </select>
<input type=text name=t1 id=TextIsHidden>
</form>
</body>
</html>