小明同学学习了比热容知识后，联想到热水袋，设计了一种水暖背心，冬天将袋内灌满热水，穿在身上可暖胸背，他设计的原理是（　　）

某地曾一度流行丙型肝炎，经查实，是由于输了不洁净的血液所致，这种不洁净的血液属于(    )

下列物质排放到空气中，会造成空气污染的是   (      )
①燃烧产生的烟；②汽车排放的尾气；③化工厂排放的废气；④石油燃烧产生的气体；
⑤动植物呼吸排出的气体

阅读理解
    China Youth says that more and more girl students have entered colleges to study in recent few years. As we know, once the number of boys was larger than that of girls, and the boys did much better than girls in our country for a long time. Now things have changed a lot.
In recent years, the proportion of girl students is growing fast. In 1998, about 39.7% of college students were girls. In 2000, it was 41.07% and in 2001, the number was 42.14%.
    There are several reasons for this.
    Firstly, there is little difference between boys and girls in their intelligence(智力). That’s to say, girls are as clever as boys. And boys have more interests than girls, so they may not do well in their study.
    Secondly, girls are more careful and more sensible than boys. They work harder at school and they do better in all exams.
    Another reason is that with the development of society, people pay more and more attention to girls' education.

已知数列{a_n}的前n项和S_n=n²+n+1，那么它的通项公式为a_n=____