Which formula can be used to describe the sequence below? 27, 9, 3, 1, 1/3, .......
a. an = -3a_n-1:a_1 = 27
b. an = 1/3a_n-1:a_1 = 27
c. an = a_n-1 -3:a_1 = 27
d. an = 3a_n-1 -1/3:a_1 = 27