图1是某同学在学习中国古代史下册时整理的部分笔记。其学习的主题是（   ）

1kg酒精完全燃烧可放出3.0×10⁷J的热量，如果完全燃烧2kg酒精，可放出____ J的热量。

根据短文理解，选择正确答案。

    There are fifty students in our class. There are twenty-four boys and twenty-six girls. Some students live near the school, and some others live far from school. About half of the students usually come to school by bike. They often get to school at a quarter to seven. About fifteen students often come to school by bus. They often get to school very early, too. Another ten students come to school on foot. Their homes are near the school, but they are often late for school, because they get up late.