【历史上重大改革回眸】阅读下列材料，回答问题。

材料一

西周的学校教育……教师既行教事，又兼管国家的日常事务，形成了“官师合一”的局面；教学场所同时也是举行祭祀、飨射、治历、望气、布政等日常国事的场所……春秋战国时期，官学赖以生存的政治、经济基础逐渐瓦解……对士的需要激增，于是平民中涌现出一批新的士，士也逐渐成为知识分子的通称。士阶层的壮大为私学的产生提供了必要的师资条件……私学是由私人授徒办学的教学组织形式……战国时期，随着社会局势的巨变和民间学术文化的发展，又有许多哲人、学者投身于教育行列，专以一家之言立教。春秋战国时期的私学……自由讲学，各具特色；为诸家学派的基地；是参议时政的论坛……游学列国，进行学术传播和交流。

——摘编自杨宁一主编《历史学习新视野新知识》

材料二

西汉初年的景帝时期，出生于今安徽的文翁来成都任蜀郡守，他“见蜀地僻陋，有蛮夷风”（《汉书•循吏传》），遂将教育文化事业放在首位……选拔青年人亲自进行教育，并派到长安去学习数年，学成之后用作各级官吏，带动各地的教化……在成都开办学校，校中学子在课堂之外还必须经常到各地有所实践。一段时间之后，效果大显，人才辈出，风气大变。“由是大化，蜀地学于京师者比齐鲁焉。至武帝时，乃令天下郡国皆立学校官，自文翁为始云。文翁终于蜀，吏民为立祠堂，岁时祭祀不绝。至今巴蜀好文雅，文翁之化也。”（《汉书•循吏传》）

——摘编自袁庭栋《天府的记忆》

北京人生活的年代距今大约多少万年（　　）

    Tim Becker and his neighbors are doing something to make their neighborhood a safe area.

    When Tim Becker gets in his car to go shopping, he doesn't1 drive to a store and back home. He always looks 2up and down the streets of his neighborhood. He looks for anything 3 such as strange cars, loud noises, 4 windows, or people gathering on street corners.

    Tim 5 to a neighborhood watch group（巡逻小组）in Strongsville, Indiana, USA. The neighborhood watch group6 on his third Wednesday of every month. That's 7 Tim gets together with about ten of his neighbors to discuss community 8 Members of the neighborhood watch group want to help the police 9 their homes, streets, and families.

    Tina Stedman,10president of neighborhood watch group, agrees with Tim. “People seem to think that crime happens to other people but not 11them. Actually, it really happens,” she said, “and I think nobody has the12 to steal from other people or to make them feel 13 at their own homes.”

    Alex, a member of the group, said that all the neighbors were14to help one another, “We15 each other's homes. We keep watch on the neighborhood at night and on weekends. Usually a16 of four or five of us goes out together. If something doesn't look right, we will call the 17 For example, if we notice a group of teenagers who seem to be looking for 18 or someone destroying property（财产）, we report to the police.”

    Alex feels the neighborhood watch group19a lot in reducing crimes. Her husband Jim agrees, “Police are good people, but they can't do20.

两个电阻R₁和R₂并联，它们并联的等效电阻为R，为什么R既小于R₁也小于R₂？