How do we solve this question?

Question 2: Let R(A,B,C,D) be a relation, where attributes C and D can be NULL, and where (A, B) is the primary key for that relation. Assume that A's domain has 8 different values, B's domain has 6 different values, C's domain has 5 different values, and D's domain has 2 different values. What is the maximum number of different tuples that can be in R