the graph of f(x) = x ^ 2 is shifted 3 units down to obtain the graph of g(x). Which of the following equations best describes g(x)?

A: g(x) = (x+3)^2
B:g(x) = x^2 -3
C: g(x) = x^2 +3
D: g(x) = (x-3)^2​