Which formula defines the sequence f(1) = 2, f(2)= 6, f(3) = 10, ƒ(4) = 14, ƒ (5) = 18?

a. f(1) = 2, f(n) = 6 + f(n-1) for n > 1
b. f(1) = 2, f(n) = 4 + f(n-1) for n > 1
c. f(1) = 2, f(n) = 2 + f(n-1) for n > 1
d. f(1) = 6, f(n) = 4 + f(n-1) for n > 1