對不起，不知道為什麽，我上麵的按編輯進去有英文，可是不按編輯看不見。我就此（裏麵）再發一次作業。謝Go兄，周末快樂！

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.