这个功能如何实现啊？？？？？？？？

<html>
<head>
<title>test</title>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
<script language="JavaScript">
<!--
function set(n)
{
    var obj
    var i    for (i=1; i<=4; i++) {
        obj = eval("form1.select" + i)
        if (i == n)
            obj.disabled = ""
        else
            obj.disabled = "yes"
    }
}
//-->
</script>
</head><body>
<form name="form1" method="post" action="">
    <table width="100%" border="1">
        <tr align="center"> 
            <td> 
                <input type="button" name="Button1" value="Button" onclick="set(1)"></td>
            <td> 
                <input type="button" name="Submit2" value="Button" onclick="set(2)"></td>
            <td> 
                <input type="button" name="Submit3" value="Button" onclick="set(3)"></td>
            <td> 
                <input type="button" name="Submit4" value="Button" onclick="set(4)"></td>
        </tr>
        <tr align="center"> 
            <td> 
                <select name="select1">
                    <option value="测试一">测试一</option>
                    <option value="测试二">测试二</option>
                    <option value="测试三">测试三</option>
                </select></td>
            <td> 
                <select name="select2">
                    <option value="测试一">测试一</option>
                    <option value="测试二">测试二</option>
                    <option value="测试三">测试三</option>
                </select></td>
            <td> 
                <select name="select3">
                    <option value="测试一">测试一</option>
                    <option value="测试二">测试二</option>
                    <option value="测试三">测试三</option>
                </select></td>
            <td> 
                <select name="select4">
                    <option value="测试一">测试一</option>
                    <option value="测试二">测试二</option>
                    <option value="测试三">测试三</option>
                </select></td>
        </tr>
    </table>
</form>
</body>
</html>