太平天国运动由盛转衰的转折点是（     ）

                                                                                          次北固山下
                                                                                                王湾
                                                                                 客路青山外，行舟绿水前。
                                                                                 潮平两岸阔，风正一帆悬。
                                                                                 海日生残夜，江春入旧年。
                                                                                 乡书何处达？归雁洛阳边。