Question 159
Two schools face each other in a chess tournament, each of them entering $75$ pupils: $75$ games are organised in which all the students play one against one (a member of the first school against one of the second), under the supervision of an external referee. Each school numbers its students from $1$ to $75$, and the referee imposes the rule that two pupils cannot play each other if the difference between their numbers is a multiple of $3$. If $n$ is the number of possible pairings satisfying this rule, how many zeros does $n$ end with?
(A) 6
(B) 12
(C) 15
(D) 18
(E) 21
(A) 6
(B) 12
(C) 15
(D) 18
(E) 21