对不起，不知道为什么，我上面的按编辑进去有英文，可是不按编辑看不见。我就此（里面）再发一次作业。谢Go兄，周末快乐！

David Card, the 2021 Nobel Prize winner in Economics, was born on a farm in Canada in 1950s, and attended a small school that had only one classroom, where a single teacher was responsible for teaching about 30 students from different grades. As a matter of fact he was attentive to most curricula which were several grades above him.