It is best to give consideration to both topics, with basic topics accounting for most and challenging topics accounting for a small part, so that students with poor foundation can solve some of them by themselves, not to give up on themselves, but also to enhance their self-confidence, and then give appropriate encouragement and guidance. A good foundation for affirmation lies in high-level topics, and a small number of challenging topics are given more challenging topics, which will make persistent efforts and will not be too confident. The explanation process is too biased towards difficult problems, so we should spend some time on basic topics.