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Question

给你段代码研究:<script>
function loadThreadFollow(t_id){
var targetImg =eval("document.followImg" + t_id);
var targetDiv =eval("document.all.follow" + t_id);
//alert(targetDiv.id);
if (targetDiv.style.display=="none"){
targetDiv.style.display="block";
targetImg.src="../images/nofollow.gif";
targetImg.title="关闭列表";
}
else
{
targetDiv.style.display="none";
targetImg.src="../images/plus.gif";
targetImg.title="展开列表";
}
}
</script>
<table width="172" border="0" cellpadding="0" cellspacing="0">
  <tr> 
    <td valign="top"><img src="../images/plus.gif" name="followImg1001" style="cursor:hand;" onclick="loadThreadFollow(1001)" title=展开列表></td>
    <td width="162" height="20"><a href="main.htm" target="_parent">信息系统介绍</a></td>
  </tr>
  <tr style="display:none" id="follow1001">
    <td>&nbsp;</td>
    <td><table border="0" cellpadding="0" cellspacing="0">
        <tr class="LEFT"> 
          <td width="15" height="18" valign="top" class="word"><img src="../images/plus.gif" name="followImg1002" style="cursor:hand;" onclick="loadThreadFollow(1002)" title=展开列表></td>
          <td height="20" class="word"><strong>第1单元 万维网</strong></td>
        </tr>
        <tr style="display:none" id="follow1002"> 
          <td height="18" class="word">&nbsp;</td>
          <td class="word">
              <table height="40" border="0" cellpadding="0" cellspacing="0">
                <tr> 
                  <td width="15" valign="top">&nbsp;</td>
                  <td height="20">1.1 使用万维网</td>
                </tr>
                <tr> 
                  <td valign="top">&nbsp;</td>
                  <td height="20"><table border="0" cellpadding="0" cellspacing="0">
                      <tr class="LEFT"> 
                        <td height="18" class="word"><a href="unit1/1.1.1.html" target="mainframe1">1.1.1 
                          万维网上冲浪</a></td>
                      </tr>
                      <tr class="LEFT"> 
                        <td height="18" class="word"><a href="unit1/1.1.2.html" target="mainframe1">1.1.2 
                          你自己的网页</a></td>
                      </tr>
                      <tr class="LEFT"> 
                        <td height="18" class="word"><a href="test1-1.htm" target="mainframe1">评估：练习一</a></td>
                      </tr>
                      <tr class="LEFT"> 
                        <td height="18" class="word"><a href="test2-1.htm" target="mainframe1">评估：考试一</a></td>
                      </tr>
                      <tr> 
                        <td class="word"></td>
                      </tr>
                    </table></td>
                </tr>
              </table>
            </td>
        </tr>
        <tr class="LEFT">
          <td height="18" valign="top" class="word"><img src="../images/plus.gif" name="followImg1003" style="cursor:hand;" onclick="loadThreadFollow(1003)" title=展开列表></td>
          <td class="word"><strong>第2单元 万维网</strong></td>
        </tr>
        <tr style="display:none" id="follow1003"> 
          <td height="18" class="word">&nbsp;</td>
          <td class="word">
              <table height="40" border="0" cellpadding="0" cellspacing="0">
                <tr> 
                  <td width="15" valign="top">&nbsp;</td>
                  <td height="20">2.1 使用万维网</td>
                </tr>
                <tr> 
                  <td valign="top">&nbsp;</td>
                  <td height="20"><table border="0" cellpadding="0" cellspacing="0">
                      <tr class="LEFT"> 
                        <td height="18" class="word"><a href="unit1/1.1.1.html" target="mainframe1">2.1.1 
                          万维网上冲浪</a></td>
                      </tr>
                      <tr class="LEFT"> 
                        <td height="18" class="word"><a href="unit1/1.1.2.html" target="mainframe1">2.1.2 
                          你自己的网页</a></td>
                      </tr>
                      <tr class="LEFT"> 
                        <td height="18" class="word"><a href="test1-1.htm" target="mainframe1">评估：练习一</a></td>
                      </tr>
                      <tr class="LEFT"> 
                        <td height="18" class="word"><a href="test2-1.htm" target="mainframe1">评估：考试一</a></td>
                      </tr>
                      <tr> 
                        <td class="word"></td>
                      </tr>
                    </table></td>
                </tr>
              </table>
            </td>
        </tr>
        <tr> 
          <td class="word"></td>
          <td class="word"></td>
        </tr>
      </table></td>
  </tr>
</table>

Answer 1

<html><head>
<title>New Page 1</title>
<script>
function check()
{
   if(f1.d1.options[1].selected)
   {
     f1.d2.length=0
     f1.d2.options.add(new Option("浙江"))
     f1.d2.options.add(new Option("江苏"))
   }
   
}
function check1()
{
  if(f1.d2.options[0].selected)
     {
       f1.d3.length=0
       f1.d3.options.add(new Option("杭州"))
       f1.d3.options.add(new Option("宁波"))
     }}
</script>
</head><body><form name=f1 method="POST" action="">  <p><select size="1" name="d1" onchange="check()">
    <option>Iran</option>
    <option>China</option>
    <option>USA</option>
    <option>Taiwan</option>
  </select><select size="1" name="d2" onchange="check1()">
  </select><select size="1" name="d3">
  </select></p>
</form></body></html>

Answer 2

http://expert.csdn.net/Expert/topic/1793/1793783.xml?temp=.3522608