In Greifswald there are three schools called A,B and C, each of which is attended by at least one student. Among any three students, one from A, one from B and one from C, there are two knowing each other and two not knowing each other. Prove that at least one of the following holds:[*]Some student from A knows all students from B.
[*]Some student from B knows all students from C.
[*] Some student from C knows all students from A. inductioncombinatorics proposedcombinatorics