At a seminar, 12 people participated in the meeting after statistics, 1 person knew all people, 1 person knew 6 people, 3 persons knew 5 people, and 5 persons knew 3 people. This shows:. ['Everyone at the meeting knew each other.', 'The chairman knows everyone.', "Some people know each other but they don't know him.", 'Some people are introduced by others.']