If the function f is continuous for all real numbers, and if F'(x) = f(x), then integral 4 , 1 f(3x)dx =

a) F(12) - F(3)
b) 3[F(4) - F(1)]
c) 1/3[F(4) - F(1)]
d) 1/3[F(12) - F(3)]