For a function f that is continuous on the closed interval [a,b], which of the following sets contains all points that must be examined to locate the absolute extrema of f on [a,b]?
a) All interior points where f′(x)=0 or f′(x) does not exist
b) All interior points where f′(x)=0, together with x=a and x=b
c) All interior points where f′(x)=0 or f′(x) does not exist, together with x=a and x=b
d) Only the endpoints x=a and x=b