Let f and g be continuous functions such that f'(x) = g'(x) for all x. Which of the following statements must be true? I f(x) = g(x) for all x. II. If f(0) = 8(0), then f(x) = g(x) for all x. III. There is a constant k such that f(x) = g(x) + k for all x . I only O III only I and II only II and III only I II, and