如何得到自定义控件里面子控件的值？

Question

<%@ Control Language="c#" AutoEventWireup="false" Codebehind="Area.ascx.cs" Inherits="WebApplicationBusiness.Uc.Area" TargetSchema="http://schemas.microsoft.com/intellisense/ie5"%>
<SELECT NAME="s1">
<option selected>
需要JavaScript才能正确显示</option>
</SELECT>
<SELECT NAME="s2">
</SELECT>
<SELECT NAME="s3">
</SELECT>
<SELECT NAME="s4">
</SELECT>
<SCRIPT LANGUAGE="JavaScript">
<!-- //选择框的名字
selector = [ "s1", "s2", "s3", "s4" ]; //选择项: "名字", "值", 子选择
menu = [
"地球","1",
 [
 "中国","86",
  [
  "北京","BJ",
   [
   "--","1",null
   ],
  "四川","SC",
   [
   "成都","28",null,
   "乐山","",null,
   "攀枝花","",null,
   "自贡","",null,
   "德阳","",null,
   "绵阳","",null
   ]
  ],
 "米国","1",
  [
  "加利福尼亚","CA",
   [
   "旧金山","1",null,
   "洛杉矶","2",null
   ],
  "华盛顿","WA",
   [
   "西雅图","seattle",null
   ]
  ]
 ],
"火星","2",
 [
 "大峡谷","86",
  [
  "小山丘","BJ",
   [
   "A地区","28",null,
   "B地区","",null,
   "C地区","",null
   ]
  ]
 ]
]; function wizz(level)
{
 if( level == 0 )
  a = menu;
 else
 {
  str = "menu";
  for( i=0; i<level; i++ )
   str += "[" + (document.all[selector[i]].selectedIndex * 3 + 2)  + "]";
  a = eval(str);
 }
 if( a == null ) return;
 
 s = document.all[selector[level]];
 i = s.length;
 while( i > 0 ) s.options[--i] = null;
 while( i < Math.floor(a.length/3) )
  s.options[i] = Option( a[i*3], a[i++*3+1] );
 s.onchange = Function("wizz(" + (level+1) + ")");
 wizz(level+1);
} wizz(0);//-->
</SCRIPT>比如我要返回得到该控件中的select的值。