Generally speaking, the ABC problem in national competitions over the years, A problem is more difficult, and it is a physics/engineering problem with strong professionalism, and the number of people selected is the least every year. Therefore, it is not recommended that Xiaobai choose the A question with high difficulty coefficient unless the major is relatively matched or the band is large; Because in the end, you may find that you can't even understand the topic, and you don't know how to start with an idea. The result is that you have scribbled for three days (three days with your eyes closed) and will hand in your paper.