At a point a, the tangent line to the graph of a differentiable function f is used to approximate nearby function values. If this linear approximation is an underestimate for x values close to a, which of the following is the best conclusion about f at x=a?
a) f′′(a)=0
b) f′′(a)>0
c) f′′(a)<0
d) f′′(a) does not exist