A number is related to order, and C number has nothing to do with order. For example, if you take two of the three balls of red, yellow and blue, A32 is equal to 6, and red, yellow and red are regarded as two situations, while C32 is 3, and red, yellow and red are regarded as the same situation. This is the difference between permutation and combination. When doing the questions, we should consider whether the questions have order requirements to distinguish the use of AC.