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Suppose $f''$ is continuous on $(-\infty,\infty)$.

If $f'(3)=0$ and $f''(3)=-2$, which of the following is true?

$f$ has a local minimum at 3

$f$ has a local maximum at 3

$f$ has a global minimum at 3

$f$ has a global maximum at 3

none of the above