jQuery AJAX级联选择菜单

Question

<script type="text/javascript">
    $(document).ready(function(){
        $("select#type").attr("disabled","disabled");
        $("select#model").attr("disabled","disabled");
        $("select#category").change(function(){
        $("select#type").attr("disabled","disabled");
        $("select#type").html("<option>Please wait...</option>");
        var id = $("select#category option:selected").attr('value');
        $.post("../select_type.php", {id:id}, function(data){
            $("select#type").removeAttr("disabled");
            $("select#type").html(data);
        });
    });
    $("select#type").change(function(){
        $("select#model").attr("disabled","disabled");
        $("select#model").html("<option>Please wait...</option>");
        var id2 = $("select#type option:selected").attr('value');
        $.post("../select_model.php", {id2:id2}, function(data){
            $("select#model").removeAttr("disabled");
            $("select#model").html(data);
        });
    });
    $("select#model").change(function(){
        var cat = $("select#category option:selected").attr('value');
        var type = $("select#type option:selected").attr('value');
        var model = $("select#model option:selected").attr('value');
        if(cat>0 && type>0 && model >0)
        {
            var model = $("select#model option:selected").html();
            var type = $("select#type option:selected").html();
            $("#result").html('<br>Your choice: ' + type + ' ' + model + '.');
        }
        else
        {
            $("#result").html("<br>Error! You did not select everything!");
        }
        return false;
    });
});

这是我的代码。它几乎正常工作......我的问题是，每当我进入选择菜单的第3级，然后将第一级更改为默认值时，其他2级将不会更改为不可用且仍可选择。我该如何扩展代码？