In 1901, the citizens of Colorado Springs in the USA decided to collect everyday items and to seal (封住）them in a special box. The box was marked “To be opened after midnight, December 31^st, AD 2000”，and was kept in the Colorado College Library.
   One hundred years later, on the appointed (约定）day, 300 people gathered to watch the opening of the box. Many in the crowd were the descendants (后代）of people who had placed things inside the box.
   When the box was opened after midnight, the things inside the box were in very good condition. There were newspapers, photos, diaries, name cards, family trees, books and lots of letters, including one written by Theodore Roosevelt, who became the President of the USA later that year. One of Roosevelt’s friends lived in Colorado Springs at that time.
   Many of the letters were written to their descendants. They describe the hopes that the people of 1901 had for the people of the next century. At that time, Colorado Springs had just a few thousand people. Now nearly half a million people live there.
   Colorado College Library has scanned the materials and put them on a website. Cecil Muller, whose grandfather had placed a collection of postcards in the box, said that the time capsule was a great treasure. “This is a wonderful educational resource(资源）.  We can learn so much about our history,”he said. “I never knew my grandfather, but now I feel very close to him. ”

机会成本是经济学原理中一个重要的概念，是指为了得到某种东西而所要放弃另一些东西的最大价值；也可以理解为在面临多方案择一决策时，被舍弃的选项中的最高价值者是本次决策的机会成本；还指厂商把相同的生产要素投入到其他行业当中去可以获得的最高收益。当下列情况出现时，你的理性选择是（    ）

①预期银行存贷款基准利率下调——买入股票

②预期通货膨胀率进一步上升——增加现期消费

③人民币对美元汇率持续跌落，对英镑汇率持续上升——到美国留学

④2016年12月21郑州市出台住房限购升级政策——投资买入新房

设数列{a_n}满足：a₁=2，a_n+1=1﹣$\frac{1}{a_n}$，记数列{a_n}的前n项之积为Π_n，则Π₂₀₁₃的为（　　）

“螳螂捕蝉，黄鹊在后”描述的是（  ）

材料一：贝克莱认为，“存在就是被感知”，事物是由人的各种感觉构成的。黑格尔认为，整个世界是“绝对观念”的“外化”和产物。

材料二：我国北宋哲学家张载提出了以“气”为核心的宇宙结构说。他认为世界是由两部分构成的，一部分是看得见的万物，一部分是看不见的，而两部分都是由“气”组成的。