Which function best represents this model? | Math SAT Question

A model estimates that in a particular forest, the number of trees with any given diameter measured at shoulder height is

21%

less for each

1

-inch increase in tree diameter measured at shoulder height. The model can be written in the form

f (x) = a b^{x}

, where

a

and

b

are constants and

x

is the tree’s diameter, in inches, measured at shoulder height, and

x \geq 5

. The model estimates that

3, 100

trees in this forest have a diameter of

13

inches measured at shoulder height.

Which function best represents this model?

f (x) = 3, 100 (0.21)^{x}

f (x) = 3, 100 (0.79)^{x}

f (x) = 66, 000 (0.21)^{x}

f (x) = 66, 000 (0.79)^{x}