Let f be a function that is continuous on the closed interval [1, 3] with f(1) = 10 and f(3) = 18. Which of the following statements must be true?
(A) 10 = f(2) 18
(B) f is increasing on the interval (1, 3).
(C) f(x) = 17 has at least one solution in the interval (1, 3).
(D) f'(x) = 8 has at least one solution in the interval (1, 3). (B) * f(x)dx > 20