题目

如图，在△ABC中，点D是BC的中点，点E、F分别是线段AD及其延长线上，且DE=DF，给出下列条件：①BE⊥EC；②BF∥EC；③AB=AC，从中选择一个条件使四边形BECF是菱形，并给出证明，你选择的条件是 （只填写序号）． 答案：③    解：∵BD=CD，DE=DF， ∴四边形BECF是平行四边形， ①BE⊥EC时，四边形BECF是矩形，不一定是菱形； ②四边形BECF是平行四边形，则BF∥EC一定成立，故不一定是菱形； ③AB=AC时，∵D是BC的中点， ∴AF是BC的中垂线， ∴BE=CE， ∴平行四边形BECF是菱形．If you see a group of people dancing and singing on the street or in the railway station, you don't need to feel surprised. They are a flash mob(暴民). Who are they? Are they mobs? Don't be confused by their name. Actually, a flash mob is a group of people who gather suddenly in a public place, do something unusual for a brief period of time, and then quickly disappear.They are usually organized with the help of the Internet or other digital communications networks. The messages may be sent to friends, who send to more people. At a predetermined time, they gather and perform some activities such as exchanging books, coming together to look at the sky, waving their hands and yelling something at the top of their voice for 30 seconds. Then, they quickly disappear before the police can arrive. Using mobile phones, the flash mob can change its place if the first one has been cancelled for any reason.Bill Wasik, senior editor of Harper's Magazine, organized the first flash mob in Manhattan in May 2003 and the first successful flash mob gathered on June 3,2003. Wasik claimed that he created the flash mob as a social experiment designed to laugh at fashion seekers and stress the cultural atmosphere of wanting to be an insider or part of “the next big thing”．Flash mob gatherings can sometimes shock people. Such an activity might seem amusing and magical, but it also might frighten people who are not aware of what is taking place. Undoubtedly, flash mob can serve as good political tools and have great potential, such as using to advertise a product.The flash mob is now becoming more and more popular. People use it to do many things. Flash mobs give people from all walks of life an opportunity to come together to create a memory.1.What is NOT the feature of the flash mob?A．It can get together quickly.B．It can change its place freely.C．It can do activities suddenly.D．It can injure people seriously.2.How do flash mobs inform one another?①By using the Internet.②By writing letters.③By yelling.④By waving hands.⑤By using mobile phones.⑥By holding a meeting.A．③④ B．①⑤ C．②⑥ D．⑤⑥3.Why did Bill Wasik create the flash mob?A．To advertise some products.B．To help people make friends.C．To laugh at fashion seekers.D．To create some memories.4.What can we learn about the flash mob from the passage?A．Bill Wasik organized the first successful flash mob in May, 2003.B．People are encouraged to take a more active part in an activity.C．Flash mobs tend to do something illegal for a short time.D．Flash mob gathering can frighten all the people present.