如图，C、D分别是一个湖的南、北两端A和B正东方向的两个村庄，CD=6km，且D位于C的北偏东30°方向上，则AB的长为（　　）

阅读材料，回答问题

材料二：

    意大利机器人“Dustbot”其实是个分类回收垃圾桶。它虽然长得很萌，行为却是很严谨的。意大利的垃圾分4类：玻璃、塑料、纸张和厨余。如果你一不小心放错了类，是会被Dustbot拒收的。

    当然，Dustbot只是一个尝试性产品，最初设计的目的是到那些卡车去不了的小巷子里收集垃圾。2009年在意大利投放试用的也只有两台而已。两年过去了，Dustbot的故事再无下文，垃圾分类仍然是耗时耗力的巨大工程。

    事实上，世界各国在处理垃圾、尤其是垃圾分类上所做出的努力远远不止研发一个机器人这么简单，更庞大的系统比比皆是。譬如，在斯德哥尔摩的汉马贝滨海新城（Hammarby），垃圾筒简直是无底洞——无论你扔进去多少垃圾，都不会满溢。而且，就算你在垃圾筒旁蹲守上一年，也逮不住一个收垃圾的人。原来，此区采用的是垃圾地下自动收集系统，类似一个巨大的地下吸尘器，垃圾们在“吸管”中以每小时70公里的速度奔向垃圾收集站，速度媲美地铁。

    与传统方式相比，这套垃圾自动收集系统效率更高：在垃圾量正常的情况下，用传统方式收集垃圾需要用150个小时，而垃圾自动收集系统仅需1．4小时，其效率是传统方式的100多倍。当垃圾量多时，这套系统的优势更加明显，1小时就能处理传统方式耗时400小时才能处理的垃圾。

    当然，这套系统是完全建立在垃圾分类的基础上的。只有垃圾在投放时分类明确，系统对垃圾的后期处理才能高效。事实上，瑞典传统垃圾分类为14类，而自动收集系统的分类则为7类。

    说到垃圾分类的源头——民众自发分类投放，日本堪称世界上规则最为严苛的国家。整个分类系统令人叹为观止：基础分类为可燃物、不可燃物、资源类、粗大类、有害类，这几类再细分为若干子分类，每个子分类又继续分为孙分类……简直是子子孙孙无穷匮也。例如横滨市，垃圾类别为10类，政府给每个市民发了长达27页的手册，其条款有518项之多：口红属“可燃物”，用完的口红管则属“小金属物”；袜子一只属“可燃物”，两只且“没被穿破、左右脚搭配”则属“旧衣料”；领带“洗过、晾干”才属“旧衣料”，否则又是“可燃物”……这还不算烦，德岛县上胜町的垃圾细分到44类，保证你每扔一次垃圾，都要受到极高的智力挑战。

下列对材料二的理解或推断正确的一项是（    ）

 "Listen carefully to what I say," the time traveler said, "I shall tell you something that you will not agree."  

 "Why will we not agree?" I asked.

 "Because the science and mathematics you know are not correct,"  he said. "You believe there are only three dimensions(三维空间)-length, breadth(宽) and thickness." 

 "There is nothing to argue with that, " the youngest member of our little group said.

 "Exactly!"  the time traveler answered, "But there is also a fourth dimension - time."   

 "We all know about time, " another member of the group said. "Time passed."

 "I do not agree,"  the time traveler said, "Length, breadth and thickness do not pass. They stay in the same place in space.  We move about them. I can prove that time is the same. It does not pass. We move in time the same way that we move in breadth, length and thickness."

 None of us believed the time traveler, and we argued with him for many hours.

 "If we could travel back in time,"  the scientist said, "We could do things to change the future." 

 "If we could travel forward in time," the doctor(博士) said, "we could do things that change our present." 

 The time traveler smiled, "It is clear," he said, "that there is only one way I can show you that I am right. We must do an experiment. Please wait here." 

 He left the room and soon returned, carrying what looked like a clock.

 "This is a model of a time machine. It took me two years to make it," the time traveler said. "I want all of you to understand that when I start the machine, it will immediately travel in time and disappear from the present." 

 He turned to the youngest of all. "Give me your hand," he said.

 The young man put out his hand. The time traveler took it and placed it on the machine. Immediately, there was a sudden wind, and the little machine disappeared.

 I was certain this was not a trick, but the doctor was not so sure.

 "Are you asking us to believe," he said, "that the machine is now in a different time?" 

 "Certainly! In my laboratory, a full-size machine is almost complete. As soon as it is complete, I shall send myself on a journey through time. Would you like to see my machine?"

 "Are you serious about this?" the doctor asked.

 "I have never been more serious about anything," the time traveler said, "Come." 

We followed him to his laboratory. There, we saw the actual machine, not quite complete, but large enough for a man to sit in.