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？
  ---------------------------------------------------------------
  
  给你一个，以此类推
  <script LANGUAGE="javascript">
  <!--
  var onecount;
  onecount=0;
  
  subcat = new Array();
  subcat[0] = new Array("徐汇区","01","001");
  subcat[1] = new Array("嘉定区","01","002");
  subcat[2] = new Array("黄浦区","01","003");
  subcat[3] = new Array("南昌市","02","004");
  subcat[4] = new Array("九江市","02","005");
  subcat[5] = new Array("上饶市","02","006");
  
  onecount=6;
  
  function changelocation(locationid)
  {
  document.myform.smalllocation.length = 0;
  
  var locationid=locationid;
  var i;
  document.myform.smalllocation.options[0] = new Option('====所有地区====','');
  for (i=0;i < onecount; i++)
  {
  if (subcat[i][1] == locationid)
  {
  document.myform.smalllocation.options[document.myform.smalllocation.length] = new Option(subcat[i][0], subcat[i][2]);
  }
  }
  
  }
  
  //-->
  </script>
  </head>
  
  <form name="myform" method="post">
  <select name="biglocation" >
  <option value="01" selected>上海</option>
  <option value="02">江西</option>
  </select>
  <select name="smalllocation">
  <option selected value="">==所有地区==</option>
  </select>
  </form>
  ---------------------------------------------------------------
  
  <script language=JavaScript>
  <!--
  ct = new Array();
  ct[0] = "none";
  //直辖市--北京 1,1
  ct[1] = "北京";
  //直辖市--上海 2,1
  ct[2] = "上海";
  //直辖市--天津 3,1
  ct[3] = "天津";
  //直辖市--重庆 4,1
  ct[4] = "重庆";
  //河北省的主要城市 5,10
  ct[5] = "石家庄";
  ct[6] = "保定";
  ct[7] = "秦皇岛";
  ct[8] = "邯郸";
  ct[9] = "承德";
  ct[10] = "廊坊";
  ct[11] = "张家口";
  ct[12] = "沧州";
  ct[13] = "邢台";
  ct[14] = "唐";
  //山西省的主要城市 15,10
  ct[15] = "太原";
  ct[16] = "长治";
  ct[17] = "大同";
  ct[18] = "临汾";
  ct[19] = "山阴";
  ct[20] = "忻州";
  ct[21] = "阳泉";
  ct[22] = "原平";
  ct[23] = "泽州";
  ct[24] = "太谷";
  //内蒙古自治区的主要城市 25,10
  ct[25] = "呼和浩特";
  ct[26] = "包头";
  ct[27] = "乌海";
  ct[28] = "临河";
  ct[29] = "东胜";
  ct[30] = "赤峰";
  ct[31] = "通辽";
  ct[32] = "锡林浩特";
  ct[33] = "乌兰浩特";
  ct[34] = "海拉尔";
  //辽宁省的主要城市";35,10
  ct[35] = "沈阳";
  ct[36] = "大连";
  ct[37] = "鞍山";
  ct[38] = "抚顺";
  ct[39] = "本溪";
  ct[40] = "丹东";
  ct[41] = "锦州";
  ct[42] = "营口";