Easy# Critical Numbers

APCALC-ZLE4SX

$x = c$ is a critical number for $f(x)$. Which of the following must be true?

I.Either $f^\prime(c)=0$ or $f^\prime(c)$ does not exist.II.The sign of the first derivative changes at $x = c$.III.$f(c)$ is a local extreme value on $f(x)$.