If \( f \) is a continuous function on \( [a, b] \), which of the following is necessarily true?
(A) \( f' \) exists on \( (a, b) \)
(B) If \( f(x_0) \) is a maximum of \( f \), then \( f'(x_0) = 0 \)
(C) \( \lim\limits_{x \to x_0} f(x) = f\left( \lim\limits_{x \to x_0} x \right) \) for \( x_0 \in (a, b) \)
(D) \( f(x) = 0 \) for some \( x \in [a, b] \)
(E) The graph of \( f \) is a straight line