Suppose / : [a, b] - R is a function with no absolute maximum on [a, b] . Which of the following must be true? @ Macmillan Learning O f is continuous on [a, b]. O f is not continuous on [a, b] . Of does not have any critical points in [a, b] . Of' (x) does not exist for some x in the open interval (a, b) . O f has no absolute minimum on [a, b]