Let A and B be two finite sets such that $$n(A - B), n(A \cap B), n(B - A)$$ are in anarithmetic progression. Here n(X) denotes the number of elements in a finite set X. If $$n(A \cup B) = 18$$, then $$n(A) + n(B)$$ is __________

A

24

B

27

C

30

D

36