If f(x) = x2 and g(x)= 3x + 1, which expression represents g(f(x))?
A) 3x^3 +x2
B) 3x^2 + 1
C) 9x^3 + 6x2 + x
D) 9x^2 + 6x + 1
E) 9x^2 + 1