Question

Let $f$ be the function defined by $f(x)=3 x^3-36 x+6$ for $-4\ \textless \ x\ \textless \ 4$. Which of the following statements is true?
$f$ is decreasing on the interval $(0,4)$ because $f^{\prime}(x)\ \textless \ 0$ on the interval $(0,4)$.
$f$ is increasing on the interval $(0,4)$ because $f^{\prime}(x)\ \textless \ 0$ on the interval $(0,4)$.
$f$ is decreasing on the interval $(-2,0)$ because $f^{\prime \prime}(x)\ \textless \ 0$ on the interval $(-2,0)$.
$f$ is decreasing on the interval $(-2,2)$ because $f^{\prime}(x)\ \textless \ 0$ on the interval $(-2,2)$.