当第一个菜单选中“a3”时，使第二个菜单失效(表单失效的方法 javascript),JavaScript

<HTML>
<HEAD >
<TITLE>Test</TITLE>
<META http-equiv=Content-Type content="text/html; charset=GBK">
<script type="text/javascript">
function ss()
{
os1=document.getElementById("s1");
os2=document.getElementById("s2");
os3=document.getElementById("s3");

if (os1.value=='a3') {
os2.disabled=true;
os3.disabled=true;}
else {
os2.disabled=false;
os3.disabled=false;}
}
</script>
</head>
<BODY>
<form id="dreamduform" action="dreamdu.php" method="post">
<label for="fav">计算机</label>
<input type="checkbox" value="1" id="fav" name="fav" />
<label for="fav">旅游</label>
<input type="checkbox" value="2" id="fav" name="fav" disabled="disabled" />
<label for="fav">购物</label>
<input type="checkbox" value="3" id="fav" name="fav" />
</form>
<form method="POST" action="123.php">
<p><select id=s1 size="1" name="D1" onChange="ss();">
<option value="a1">a1</option>
<option value="a2">a2</option>
<option value="a3">a3</option>
</select><select id=s2 size="1" name="D2">
<option value="b1">b1</option>
<option value="b2">b2</option>
<option value="b3">b3</option>
</select><select id=s3 size="1">
<option value="c1">c1</option>
<option value="c2">c2</option>
<option value="c3">c3</option>
</form>
</BODY>
</HTML>