Question 7
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 $a$ from $A$, $b$ from $B$, and $c$ from $C$ there are two knowing each other and two others not knowing each other. Prove that either some student from $A$ knows all students from $B$, or some student from $B$ knows all students from $C$, or some student from $C$ knows all students from $A$.