“停课闹革命”、“大串联”、“上山下乡”。这些现象出现的原因是（    ）

阅读诗词，解答问题。
                                                                                                          丑奴儿·书博山道中壁
                                                                                                                    （宋） 辛弃疾
                                          少年不识愁滋味，爱上层楼。爱上层楼，为赋新词强说愁。   而今识尽愁滋味，欲说还休。欲说还休，却道天凉好个秋。