白鳍豚，在长江里大约生活了2500万年的白鳍豚，是中新世及上新世延存至今的古老孑遗生物。白鳍豚是鲸类家族中小个体成员，是世界上现有5种淡水豚（拉河豚、亚河豚、恒河豚、印河豚、白鳍豚）中存活头数最少的一种。由于人类活动增加或活动不当，使白鳍豚意外死亡事故增多。据统计，1973～1985年间，共意外死亡59头，其中被鱼用滚钩或其他渔具致死29头，占48.8%；被江中爆破作业致死11头，占18.6%；被轮船螺旋桨击毙12头，占20%；搁浅死亡6头，占10%；误进水闸1头，占1.6%。另据统计，长江下游水域中意外死亡的白鳍豚，有三分之一是 被轮船螺旋桨击毙的。

根据短文选择正确的一项。

    Do you know about Xiong'an? It's a new city near Baoding. There is a big lake in Xiong'an. Yes, it is Baiyangdian Lake. It is very big. In summer, you will see many beautiful lotus flowers on the lake. Some are red. Some are pink. Many people like them. There will be many boats on the lake. And there will be many birds singing in the grass. Xiong'an is beautiful and new. You can come here by bus from Baoding. And you can come here by train. You will play in the water with your friends. You will eat delicious fish. You will take many wonderful pictures. You will have a nice trip in Xiong'an.

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如图所示的古希腊历史人物对当时雅典政治的最大贡献是（    ）

关于x的一元二次方程（a+c）x²+2bx+（a﹣c）=0，其中a、b、c分别为△ABC三边的长．