For a continuous and differentiable function f defined on the closed interval [a,b], which of the following sets of points must be examined to locate its absolute maximum and minimum values?
a) Only the points where f′(x)=0 in (a,b)
b) All points where f′(x)=0 or f′(x) is undefined in (a,b)
c) All points where f′(x)=0 or f′(x) is undefined in (a,b), together with x=a and x=b
d) All points where f′′(x)=0 in (a,b), together with x=a and x=b