For which of the following function does f(x) = |f(x)| for every value of x? Explain as well.

A: f(x) = 3 - x
B: f(x) = 2x + [tex]x^2[/tex]
C: f(x) = [tex]x^3[/tex] + 1
D: f(x) = [tex]x^2 + (2-x)^2[/tex]