10. Four people, A, B, C, and D, are competing in a table tennis tournament, where each pair of players will play one match. In the end, A defeated D, and A, B, and C won the same number of matches. How many matches did D win?
10. 【Solution】Since a total of six matches were played, and "Jia, Yi, and Bing won the same number of matches," they either each won one match or each won two matches. If Jia, Yi, and Bing each won one match, Ding should have won three matches, but since Ding has already lost to Jia, it is impossible for him to win three matches. Therefore, it can only be that Jia, Yi, and Bing each won two matches, $3 \times 2=6$, the three of them together won six matches, so Ding did not win any matches.