For a differentiable function f, the derivative is given by
f′(x)=x2−93x.
Which statement best describes the behavior of f at x=3?
a) f has a vertical tangent line at x=3.
b) f has a horizontal tangent line at x=3.
c) f is not continuous at x=3.
d) f is differentiable at x=3 and f′(3)=0.