For a continuous differentiable function f on the closed interval [a,b], the absolute maximum and minimum values of f on the interval can occur at which of the following points?
a) Only at interior points where f′(x)=0
b) At any point where f′′(x)=0
c) At any interior critical point where f′(x)=0 or at the endpoints x=a or x=b
d) Only at the endpoints x=a or x=b