Marking the key points is a better choice. The first is because of the source of the problem here. Most of the questions here are the real questions of the exam, which are very representative. What's more, a thoughtful answer is given here. You can open this answer at any time if you encounter a question that you can't answer, or if you want to modify the answer after completing the question. In that case, you will learn some knowledge points and some flexible skills from the answers, which can help you in the formal exam.