Let h be differentiable on [2,6] with h(2)=5 and h(6)=21. Which of the following statements must be true?
a) There exists a c in (2,6) for which h′(c)=4.
b) For every x in (2,6), h′(x)=4.
c) There exists a c in (2,6) for which h′(c)=2.
d) h′(x)≤4 for all x in (2,6).