select控件和input控件联动的问题

<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title>无标题页</title>
    <script language="javascript" type="text/javascript">
        <!--
        function setvalue()
        {
            var txt = document.getElementById("testtext");
            var sel = document.getElementById("sel");
            txt.value = sel.options[sel.selectedIndex].text;
        }
        -->
    </script>
</head>
<body>
    <form id="form1" runat="server" action="Default2.aspx">
    <div> 
        <input type="text" id="testtext" />
        <select id="sel" onchange="javascript:setvalue();">
            <option value="1">aaa</option>
            <option value="2">bbb</option>
            <option value="3">ccc</option>
        </select>
    </div>
    </form>
</body>
</html>