嵌套的在GridView里的radiobutton 怎么才能只选中一个

//写在Aspx前台中的代码

<script type="text/javascript">
        var last = null;//最后访问的RadioButton的ID

        function judge(obj)
        {
            if(last == null)
            {
                last = obj.id;
           // alert(last);

            }
            else
            {
                var lo = document.getElementById(last);
                lo.checked = "";
         // alert(last + " " + lo.checked);

                last = obj.name;
            }
            obj.checked = "checked";
        }
    </script>

'后台的代码（VB.Net）

Protected Sub GridView1_RowDataBound(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.GridViewRowEventArgs) Handles GridView1.RowDataBound

        If e.Row.RowType = DataControlRowType.DataRow Then

            Dim rb As RadioButton = CType(e.Row.FindControl("rdo"), RadioButton)
            rb.Attributes.Add("onclick", "judge(this)")
        End If
    End Sub