Three terms of an arithmetic sequence are shown below. Which recursive formula defines the sequence?
f(1) = 6, f(4) = 12, f(7) = 18

a) f (n + 1) = f(n) + 6
b) f (n + 1) = 2f(n)
c) f (n + 1) = f(n) + 2
d) (n + 1) = 1.5f(n)