Upgrade subject to access all content

What are the critical numbers for the following function?

$$f(x) = \cfrac{1}{3}x^{3}-x^{2}-8x$$

$x=4 \text{ and } x=\cfrac{1}{2}$

$x=-4 \text{ and } x=2$

$x=4 \text{ and } x=-2$

$x=\cfrac{1}{4} \text{ and } x=2$