直接刺激沙皇亚历山大二世进行改革的是（ ）

正八边形地板砖，能铺满地面，既不留下一丝空白，又不相互重叠吗？请说明理由．

【诗歌天地】阅读下面的现代诗，完成后面的题目。
                                                                                      黄河颂　（歌词节选）
　　                                                                                 我站在高山之巅，
　　                                                                                 望黄河滚滚，
　　                                                                                 奔向东南。
　　                                                                                 惊涛澎湃，
　                                                                                 　掀起万丈狂澜;
　                                                                                 　浊流婉转，
　                                                                                 　结成九曲连环;
　                                                                                 　从昆仑山下
　　                                                                                 奔向黄海之边;
　　                                                                                 把中原大地
　　                                                                                 劈成南北两面。
　                                                                                 　啊!黄河!
　                                                                                 　你是中华民族的摇篮!
　　                                                                                 五千年的古国文化，
　                                                                                　从你这儿发源;
                                                                                　　多少英雄的故事，
                                                                                      在你的身边扮演!

若a＜0，则（　　）