If f(x) = 3x + 2 and g(x) = x^2 + 1, which expression is equivalent to (f . g)(x)?
a. (3x + 2)(x^2 + 1).
b. 3x^2 + 1 + 2
c. (3x + 2)^2 + 1.
d. 3(x^2 + 1) + 2.