Which of the following best describes a point of inflection of the graph of a twice-differentiable function f?
a) A value of x at which f′′′(x)=0
b) A value of x at which f′′(x)=0
c) A value of x at which f′′(x) equals 0 or is undefined and the concavity of f changes
d) A value of x at which f′(x)=0 and f′′(x)<0