F(x) and g(x) -- Solving for functions

23. If f(g(x)) = sqrt(x^3 + 4) , then f(x) and g(x) are

a. g(x) = x^3 and f(x) = x + 4
b. g(x) = x^3 and f(x) = sqrt(x+4)
c. g(x) = sqrt(x+4) and f(x) = x^3
d. g(x) = sqrt(x) + 4 and f(x) = x^3
e. g(x) = sqrt(x) and f(x) = x^3 + 4

Problem: http://i43.tinypic.com/k2mwt4.jpg

Find a function for g(x) so when you substitute g(x) for x in f(x), it comes out to (x^3 + 4)^(1/2)

B is the answer as when you take x^3 in g(x) and replace it for x in f(x), it comes out to (x^3 + 4)^(1/2)