直角三角形两条直角边长分别是5和12,则第三边上的中线长为(       )

        Tom and Fred were talking about the year 2050.
　　“What will 1 be like in the year 2050?” asked Tom.
　　“I don't know,” said Fred. “What do you think?”
　　“Well, no one knows. But it is 2 to guess,” said Tom. “In the year 2050 everybody will carry a pocket (袖珍) 3. The computer will give people the 4 to all their problems. We shall all have telephones in our pockets,5 . And we'll be able to talk to our friends all over the world. Perhaps(也许) we’ll be able to 6 them at the same time. Machines will do 7of the work, and people will have more8. Perhaps they will work only two or three days a week. They will be able to fly to the moon by spaceship and spend their holidays there.”
　　“I'm very 9to hear that. I hope to fly to the moon. And I hope I'll be able to live 10,” said Fred. “Won't that be interesting? Just like a fish.”

材料一：1861年俄历2月19日，沙皇亚历山大二世签署法令（又称二一九法令）。法令规定：农奴获得人身自由，地主不能买卖或交换农民，不能随意干涉农民的生活。农奴在获得“解放”时，可以得到一块土地，但必须高价赎买。

材料二：同样处于危机之下的日本，也走上了变革的道路。1868年，明治政府开始进行资产阶级性质的改革，明治政府大力兴建铁路、工厂，发展邮政和电讯事业，还积极扶持私人企业，向西方选派留学生，高薪聘请西方专家和技师到日本工作；效法西方军事制度，实行征兵制，建立常备军。

——整理自岳麓版教科书《世界历史》

材料三：1896年，梁启超在《变法通议》中曾写道：同治初年（19世纪60年代初），普鲁士首相俾斯麦对人说：“三十年后，日本其兴，中国其弱乎？日人之游欧洲者，讨论学业，讲究官制，归而行之；中国人之游欧洲者，询某厂船炮之利，某厂价值之廉，购而用之。强弱之源，其在此乎？”