Write a function to represent the geometric sequence 6, 18, 54, . . . .

A. f (x) = 6(3)^x
B. f (x) = 6(3)^x - 1
C. f (x) = 3(6)^x - 1
D. f (x) = 3(6)^x

D. f (x) = 3(6)^x