1. Complex topic design: The topic design of Li Lin's four volumes is often complicated, involving the comprehensive application of multiple knowledge points. This requires candidates to have high thinking ability and problem-solving ability, and be able to use what they have learned flexibly to solve complex problems.

2. Difficult topics: The topics of Li Lin's four volumes often involve more in-depth knowledge points to examine students' ability to understand and apply knowledge. This requires candidates to study and understand the knowledge points in depth during the preparation for the exam, and to be able to use knowledge flexibly to solve problems.