与下段文字衔接自然、连贯、得体的一句是（    ）

       财富是什么？商人摆弄着手机说，财富就是赚来的金钱；学者推了推眼镜说，财富就是创造的知识；哲人拍了拍地球仪说，财富就是拥有的智慧；农民望了望金黄的麦浪说，           ……不同的人，不同的经历，得出了不同的答案。

如图，△ABC是等边三角形，P为△ABC内部一点，将△ABP绕点A逆时针旋转后能与△ACP′重合，如果AP=3，求PP′的长．

⑴对任意a，b∈R，a*b=b*a；

⑵对任意a∈R，a*0=a；

⑶对任意a，b∈R，（a*b）*c=c*（ab）+（a*c）+（c*b）﹣2c．

关于函数f（x）=（e^x）* $\frac{1}{e^x}$ 的性质，有如下命题：

⑴f（x）为偶函数；

⑵f（x）的x=0处取极小值；

⑶f（x）的单调增区间为（﹣∞，0；

⑷方程f（x）=4有唯一实根．

其中正确的命题的序号是____．

  “Cool" is a word with many meanings．Its old meaning is used to express a temperature that is a little bit cold．As the world has changed，the word has had many different meanings．
  “Cool"can be used to express feelings of interest in almost everything When you see a famous car in the street，maybe you will say，“It's cool." You may think，“He's so cool," when you see your favorite footballer．
    We all maximize（扩大） the meaning of “cool". You can use it instead of many words such as “new" or "surprising"．Here's an interesting story we can use to show the way the word is used．A teacher asked her students to write about the waterfall（瀑布）they had visited．On one student's paper was just the one sentence, “It's so cool．Maybe he thought it was the best way to show what he saw and felt. But the story also shows a lack of words．Without “cool”，some people have no words to show the same meaning．So it is quite important to keep some credibility（可信性）．Can you think of many other words that make your life as colorful as the word "cool"？ I can．And I think they are also very cool．