For any set S let n(S) denote the number of elements in S. If A, B and C are sets such that
$$n(A \cup B \cup C) = 31, n(A) = 20, n(b) = 16, n(C) = 8, n(A \cap B \cap C') = 3, n(A' \cap B \cap C) = 2$$ and $$n(A \cap B' \cap C) = 4$$ then $$n(A \cap B \cap C) =$$

A

4

B

3

C

2

D

1