Asap

a population of bacteria contains 120 bacteria in generation 1 and triples in
population every generation. this sequence represents the number of
bacteria for the first few generations:
120, 360, 1080
what is the explicit formula for the number of bacteria in generation n?

a. a[n) = 120x3^n-1
b. a(n) = 3x120^n-1
c. a(n) = 3 + (n - 1)120
d. a(n) = 120 + (n - 1)3