Which of the following best describes a critical number of a function f that is defined on an open interval (a,b)?
a) A value c in (a,b) at which f′(c)=0
b) A value c in (a,b) where f(c)=0
c) A value c in (a,b) at which f′(c)=0 or f′(c) does not exist, provided f is defined at c
d) A value c in (a,b) where f′′(c)=0 and f is continuous at c