如图1所示的电路，电源电压保持不变，电阻R₁=10Ω．闭合开关S，变阻器滑片P从最右端滑到最左端的过程中，电压表示数U与电流表示数I的变化关系如图2中ab线段所示，由此可知：电源电压为　____ 　 V，变阻器R₂的最大阻值是　____ 　Ω．

    【甲】亲贤臣，远小人，此先汉所以兴隆也；亲小人，远贤臣，此后汉所以倾颓也。先帝在时，每与臣论此事，未尝不叹息痛恨于桓、灵也。侍中、尚书、长史、参军，此悉贞良死节之臣，愿陛下亲之信之，则汉室之隆，可计日而待也。

    臣本布衣，躬耕于南阳，苟全性命于乱世，不求闻达于诸侯。先帝不以臣卑鄙，猥自枉屈，三顾臣于草庐之中，咨臣以当世之事，由是感激，遂许先帝以驱驰。后值倾覆，受任于败军之际，奉命于危难之间，尔来二十有一年矣。

    【乙】包拯，字希仁，庐州合肥人也。始举进士，徙知端州，迁殿中丞。端土产砚，前守缘贡^①率取数十倍以遗权贵。拯命制者才足贡数^②，岁满不持一砚归。

    旧制，凡讼诉不得径造庭下。拯开正门，使得至前陈曲直，吏不敢欺。

    拯性峭直^③，恶吏苛刻，务敦厚，虽甚嫉恶，而未尝不推以忠恕也。与人不苟合，不伪辞色悦人，平居无私书，故人、亲党皆绝之。虽贵，衣服、器用、饮食如布衣时。尝曰：“后世子孙仕宦，有犯赃者，不得放归本家，死不得葬大茔中。不从吾志，非吾子若孙也。”

【注释】①缘贡：趁着进项的机会。②才足贡数：仅仅满足进项数量。③峭直：严厉刚直。

    Britain's smallest school was closed down because its only pupil failed to turn up for class, a famous Britain newspaper reported in May.
The newspaper said the six-year-old girl's parents were unhappy with a teacher who just got the new job to teach the only pupil.
The school had been closed for the last nine months after its former（前任的）teacher retired and the only other pupil moved on to a secondary（中等的）school. The new teacher, Ms. Puckey was to start teaching the girl and reopen（再次打开）the school.
But the girl's mother is keeping her daughter at home.
    “I was not pleased with the new teacher,”the mother said. “I had told the old teacher as far back as last September that if Ms. Puckey got the job, my child would not be going to school.”
    The school lies on an island off the northeast of Scotland. Although there was only one pupil, the school is very good in many ways and has a headmaster, three computers, a television, and an art room as well as a school house with three bedrooms.

拉丁美洲面积最大、人口最多的国家是(    )