We can fill in the gaps. Teachers can design different levels of exercises according to their actual situation in the classroom, so that students can choose their own problems according to their actual situation. For example: The students are in the middle class, but the base of the last 15 students is much worse than the first 30 students. In the arrangement of homework, we can label each problem when we design exercises, sub-basic questions (must do questions) , top questions (optional do questions) .