Which of the following best describes a critical point of a real-valued function f defined on an interval?
a) A number c such that f′′(c)=0
b) A number c in the domain of f where f′(c)=0 or f′(c) does not exist
c) A number c in the domain of f where f′(c)=0
d) A number c where f′(c)>0 for all nearby x