JS选择一个列表框，赋值到另一个列表框

<script >
<!--
function onselectchange(){
  num=document.send.class0.options[document.send.class0.selectedIndex].value;
  switch(num){
     case "0":
document.send.class1.options.length=4;
document.send.class1.options[00].text="A1";
document.send.class1.options[00].value=0;
document.send.class1.options[01].text="A2";
document.send.class1.options[01].value=1;
document.send.class1.options[02].text="A3";
document.send.class1.options[02].value=2;
document.send.class1.options[03].text="A4";
document.send.class1.options[03].value=3;
break;
case "1":
document.send.class1.options.length=6;
document.send.class1.options[00].text="B1";
document.send.class1.options[00].value=0;
document.send.class1.options[01].text="B2";
document.send.class1.options[01].value=1;
document.send.class1.options[02].text="B3";
document.send.class1.options[02].value=2;
document.send.class1.options[03].text="B4";
document.send.class1.options[03].value=3;
document.send.class1.options[04].text="B5";
document.send.class1.options[04].value=4;
document.send.class1.options[05].text="B6";

break;  }}
//-->
</script>

<table width="400" border="1" cellspacing="0" cellpadding="0" align="center" bordercolordark="#9cc384" bordercolorlight="#FFFFFF">
  <tr bgcolor="#E6F0DD" align="center">
    <td> <br>
      <form method="post" action="a_list.asp" name="send" target="_blank">
        <table width="62%" border="0" cellspacing="2" cellpadding="2">
          <tr>
            <td width="44%" align="right">类别：</td>
            <td width="56%">
              <table width="95%" border="0" cellspacing="0" cellpadding="0">
                <tr>
                  <td>
                    <select name="class0" onChange="onselectchange()">
                      <option value="2">** 选择一级类别 **</option>
                      <option value="0">A</option>
                      <option value="1">B</option>                      
                    </select>
                  </td>
                </tr>
                <tr>
                  <td>&nbsp;</td>
                </tr>
                <tr>
                  <td>
                    <select name="class1">
                      <option value="1">** 选择二级类别 **</option>
                    </select>
                  </td>
                </tr>
              </table>
            </td>
          </tr>
        </table>
      </form>
      <br>
      <br>
    </td>
  </tr>
 
</table>