DropDownList调用javascript控制表格行tr的显示与隐藏

DropDownList调用javascript控制表格行tr的显示与隐藏
<mce:script language="javascript"><!--
function cao() {
if(document.all.ddl_class.value=="0")
{
hongtou.style.display="none";
wailai.style.display="none";
wailai1.style.display="none";
}
else if(document.all.ddl_class.value=="1")
{
hongtou.style.display="block";
wailai.style.display="none";
wailai1.style.display="none";
}
else
{
hongtou.style.display="none";
wailai.style.display="block";
wailai1.style.display="block";
}
}
// --></mce:script>
<asp:DropDownList ID="ddl_class" runat="server" onchange="cao()">
<asp:ListItem Value="0">新建文件</asp:ListItem>
<asp:ListItem Value="1">红头文件</asp:ListItem>
<asp:ListItem Value="2">外来文件</asp:ListItem>
</asp:DropDownList>
<table>
<tr id="hongtou" style="DISPLAY:none" mce_style="DISPLAY:none">
<td>份数:</td>
<td></td>
</tr>
<tr id="wailai" style="DISPLAY:none" mce_style="DISPLAY:none">
<td>来文（电）单位:</td>
<td></td>
</tr>
<tr id="wailai1" style="DISPLAY:none" mce_style="DISPLAY:none">
<td>联系人:</td>
<td></td>
</tr>
</table>