蚊蝇等害虫对DDT产生抗药性的主要原因是（　　）。

阅读理解

    Good morning．Welcome to our school!

    My name is Hou Li，a history teacher.I'm your guide today．Look!Our big playground lies in the middle of the school．On the left of it are two black classroom buildings．There are thirty classrooms in each building．We use computers to help with teaching．It's quick and saves much time．We also put important information on the Internet．so our students can read it at home．Of course,they do their homework on the computers too，just like your students in America．We're going to join an IT lesson at nine o'clock there．

    On the right are three buildings：the office building,the dining hall and the library．The dining hall is orange．Six hundred students can eat in it at a time．Today we are going to have lunch there．The library is a popular place．There are many kinds of magazines and newspapers and hundreds of books for students and teachers．

    Let's have a rest near the office building and then go to the IT lesson．

根据材料内容选择最佳答案。

①与散文家林非先生聊天是一件很幸福的事，他语气随和，娓娓而谈，我感到无拘无束，心情非常愉快。

②打击盗版软件、保护知识产权无可非议，但采取黑屏这一方式就过了头，这样做不利于软件事业的发展。

③在世界经济徘徊不前、企业间竞争异常激烈的情形下，没有什么企业联盟颠扑不破，所谓的价格联盟也不例外。

④王旭明做客杭州的行程排得很满，记者的采访只能见缝插针地安排在他从宾馆前往浙江人文大讲堂的路上。

⑤为了建设和谐社会，城市学校应主动和乡镇学校结对子，鼎力相助薄弱学校。

⑥私立学校虽然缺乏教学管理经验，但可以向公办学校学习，可以在亦步亦趋的基础上，渐渐走出自己的路来。