赏析诗歌，回答下列各题。
                                                                                                      行路难  (李白)
                                                                                    金樽清酒斗十千，玉盘珍羞直万钱。
                                                                                    停杯投箸不能食，拔剑回顾心茫然。
                                                                                    欲渡黄河冰塞川，将登太行雪满山。
                                                                                    闲来垂钓碧溪上，忽复乘舟梦日边。
                                                                                    行路难！行路难！多歧路，今安在？
                                                                                     长风破浪会有时，直挂云帆济沧海。

如图表示一定质量的KClO₃和MnO₂固体混合物受热过程中，某变量y随时间的变化趋势，纵坐标表示的是（　　）