A paper, the top students will not find it too easy, and the poor students will find it difficult, then it is not a good grade.
The reason why intermediate students think that this question is simple is because they can't find the key to this question and can't find the loopholes in it, so they make seemingly simple mistakes.
Some seemingly simple, but error-prone questions, have a lower score than those that seem to be complex. The reason why such mistakes occur is not because they are careless, but because they do not understand the mistakes enough and are misled by other things.
The teacher's wrong explanation usually gives the student a situation where he understands himself, but will still do it wrong next time, not because they are careless, but because they have not mastered what they have learned.
An ordinary student does not expand his thinking when solving a problem, so when solving a problem, they are not a rigorous argument, but start using it without verification, and they think that they can use it, or even have obtained it, so when solving the problem, they are not so rigorous.
An ordinary student may do the easiest two steps and get a result, that is, two or three points, but they think they have answered correctly, and those geniuses, although they are all European solutions, got seven or eight percent of the scores, but they also know that they are wrong, so they think that they are not doing well.
When doing questions, ordinary students will not be like those poor students, they don't know how to answer difficult problems, and they will not be like those top students, who can always get high scores and control, but they will feel that they are doing it right, but in fact, they are doing it wrong, so they will have a sense of superiority when they take the exam.