表单中，第一个列表框有a,b,c三个选项，当用户选中a时，第二个列表框显示a1,a2,a3三个选项，（若用户在第一个列表框中选择b，第二个列表框显示b1,b2,b3,以此类推），当用户在第二个列表框中选择a1，第三个列表框中显示a11,a12,a13。请问如何实现？
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给你一个，以此类推
1<script language="javascript">
2<!--
3var onecount;
4onecount=0;
5
6subcat = new Array();
7subcat[0] = new Array("徐汇区","01","001");
8subcat[1] = new Array("嘉定区","01","002");
9subcat[2] = new Array("黄浦区","01","003");
10subcat[3] = new Array("南昌市","02","004");
11subcat[4] = new Array("九江市","02","005");
12subcat[5] = new Array("上饶市","02","006");
13
14onecount=6;
15
16function changelocation(locationid)
17{
18document.myform.smalllocation.length = 0;
19
20var locationid=locationid;
21var i;
22document.myform.smalllocation.options[0] = new Option('====所有地区====','');
23for (i=0;i < onecount; i++)
24{
25if (subcat[i][1] == locationid)
26{
27document.myform.smalllocation.options[document.myform.smalllocation.length] = new Option(subcat[i][0], subcat[i][2]);
28}
29}
30
31}
32
33//-->
34</script>
1<form method="post" name="myform">
2<select name="biglocation" onchange="changelocation(document.myform.biglocation.options[document.myform.biglocation.selectedIndex].value)">
3<option selected="" value="01">上海</option>
4<option value="02">江西</option>
5</select>
6<select name="smalllocation">
7<option selected="" value="">==所有地区==</option>
8</select>
9</form>
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1<script language="JavaScript"></script>