6.50 In an exam, there are 4 multiple-choice questions, each with 3 options. A group of students take the exam, and the result is that for any 3 people, there is 1 question where their answers are all different. How many students can participate in the exam at most?
[Solution] First, consider the following problem: If there must be 3 students among the group such that for each question, at least 2 of the 3 have the same answer, what is the minimum number of students who should participate in the exam?

Obviously, as long as there are 4 people participating in the exam, it can be ensured that two people have the same answer to the first question. Secondly, as long as there are 5 people participating, it can be guaranteed that 4 people have only two different answers to the second question, thus 3 people can be selected so that their answers to the first and second questions have at most two different answers. Furthermore, by the pigeonhole principle, as long as there are 7 people participating, it can be guaranteed that 5 people have only two different answers to the third question. Finally, by the pigeonhole principle, as long as there are 10 people participating in the exam, it can be ensured that 7 people have only two different answers to the fourth question. Thus, 3 people can definitely be selected whose answers to each question have at most two different answers. This indicates that the number of people participating in the exam is at most 9.

For the case where 9 people participate in the exam, the following table gives a satisfactory answer sheet scheme: In summary, the maximum number of people participating in the exam is 9.