A scientist is experimenting with bacteria in his laboratory. He starts out with 3 bacteria. The number of bacteria quadruple every hour. Which equation represents the growth of the bacteria over x hours?

a. f(x) = 4(3^x)
b. f(x) = 3^x + 4
c. f(x) = 4^x + 3
d. f(x) = 3(4^x)