81, 27, 9,

find the common ratio of the given sequence, and write an exponential function which represents the sequence. use n = 1, 2, 3,

a) 3; f(n) = 81^n-1

b) 3; f(n) = 81(3^)n-1

c) 1/3; f(n)=81(3)^n-1

d) 1/3; f(n)=81(1/3)^n-1