I am unable to insert data method using jquery ajax json call

<script type="text/javascript">
          $(function () {
              $('#btnSubmit').click(function () {
                  $.ajax({
                      type: "POST",
                      contentType: "application/json; charset=utf-8",
                      url: "WorkingWithAjax.aspx/InsertData",
                      dataType: "json",
                      data: "{'FirstName':'" + $('#txtFirstName').val() + "','LastName':'" + $(
                          '#txtLastName').val() + "','City':'" + $('#txtCity').val() + "','EmailID':'" + $('#txtEmailID')
                          .val() + "'}",
                      success: function (data) {
                          var obj = data.d;
                          if (obj == 'true') {
                              $('#txtFirstName').val('');
                              $('#txtLastName').val('');
                              $('#txtCity').val('');
                              $('#txtEmailID').val('');
                              $('#lblmsg').html('Data Inserted Successfully');
                          }
                      },
                      error: function (result) {
                          alert(result);
                      }
                  });
              });
          });
    </script>

[WebMethod]
        public static string InsertData(string FirstName, string LastName, string City, string EmailId)
        {
            string retMessage = string.Empty;
            string ConnectionString = "server=.\\SQLSERVER;uid=sa;pwd=Acer@123;database=data";
            using (SqlConnection con = new SqlConnection(ConnectionString))
            {
                string query = "INSERT INTO sample(FirstName,LastName,City,EmailId) values(@FirstName,@LastName,@City,@EmailId)";
                using (SqlCommand cmd = new SqlCommand(query, con))
                {
                        con.Open();
                    cmd.Parameters.Add("@FirstName", FirstName);
                    cmd.Parameters.Add("@LastName", LastName);
                    cmd.Parameters.Add("@City", City);
                    cmd.Parameters.Add("@EmailID", EmailId);

                    int AffectedRow = cmd.ExecuteNonQuery();
                    if (AffectedRow == 1)
                    {
                        retMessage = "true";
                    }
                    else
                    {
                        retMessage = "false";
                    }

                    return retMessage;

                }
            }
        }