一个下拉框来控制别一个下拉框

对不起大家拉,我现在已经作出来拉.
我现在把代码给写出来,希望能对大家有点用处.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
<title>测试页面</title>
<script language="JavaScript" defer>
var arr=["aa","bb","cc","dd"];
var arr2=[["aa1","aa2"],["bb1","bb2"],["cc1","cc2","cc3","cc4"],["dd1","dd2"]];
for(i=0;i<arr.length;i++)
{
form1.t1.options[i] = new Option(arr[i]);
}
function checkit(num)
{
form1.t2.innerHTML="";
for(i=0;i<arr2[num].length;i++)
{
form1.t2.options[i] = new Option(arr2[num][i]);
}
}
</script>
</head>
<body onLoad="checkit(0)">
<form name="form1" method="post" action="">
<select name="t1" id="t1" onChange="checkit(selectedIndex)">
</select>
<select name="t2" id="t2">
</select>
</form>
</body>
</html>