Let f(x)=x4−8x3+18x2. Which of the following best describes the nature of its critical points?
a) x=0 is a local maximum, and x=3 is a local minimum.
b) x=0 is a local minimum, and x=3 is not a local extremum.
c) x=0 is a local minimum, and x=3 is a local maximum.
d) Neither x=0 nor x=3 is a local extremum, because f′(x) does not change sign at either point.