js调用web service函数问题

    <title>Untitled Page</title>
    <script language = "javascript">
        function button_OnClick()
        {
            requestValue = CarService.GetCarValue
            (document.getElementById("textbox1").text,document.getElementById("textbox2").text,document.getElementById("textbox3").text,OnComplete,OnTimeOut);
            return false;
        }
        
        function OnComplete(result)
        {
            alert("Car is worth:$" + result);
        }
        
        function OnTimeOut(result)
        {
            alert("Timeout:" + result);
        }
    </script>
 
 
 
</head><body >
   <form id="form1" runat="server">
            
     
     
       <asp:ScriptManager ID="ScriptManager1" runat="server">
        <Services>
            <asp:ServiceReference Path = "~/CarService.asmx" />
        </Services>
       </asp:ScriptManager>
           <label id = "label1" runat = "server">Make:</label>
       <asp:TextBox ID = "textbox1" runat = "server"></asp:TextBox>
       <br />
       <label id = "label2" runat = "server">Model:</label>
       <asp:TextBox ID = "textbox2" runat = "server"></asp:TextBox>
       <br />
       <label id = "label3" runat = "server">Year:</label>
       <asp:TextBox ID = "textbox3" runat = "server"></asp:TextBox>
       <br />
    
       <input type = "button" id = "button" runat = "server" onclick = "button_OnClick();" value = "Get Value" />
以上为Default.aspx代码
   [WebMethod]
    [ScriptMethod]
    public static int GetCarValue(string strCarMake,string strCarModel,int strCarYear)
    {
        int nReturn = 0;
        if (strCarMake == "Honda")
            nReturn = 30000;
        else
            nReturn = 20000;
        if (strCarModel == "Polot")
            nReturn += 10000;
        int nDepreciation = (System.DateTime.Now.Year - strCarYear) * 2000;
        nReturn -= nDepreciation;
        return Math.Max(0, nReturn);
    }
以上为CarService.cs代码(web service函数)现在报的错是Default.aspx中 requestValue = CarService.GetCarValue这个函数CarService未定义
怎么解决