Let $f(x)=a x^{3}+b x^{2}+c x+d(a, b, c, d, \alpha, \beta \in R)$ and
$\mathrm{f}^{\prime}(\alpha)=\mathrm{f}^{\prime}(\beta)=0,(\alpha \neq \beta)$, then
(A) if $f(\alpha), f(\beta)<0$ then all the roots of $f(x)=0$ are real
(B) if $f(\alpha), f(\beta)>0$ then all the roots of $f(x)=0$ is real
(C) if $\alpha$ is a point of local maxima and $\beta$ is point of local minima for the function $f(x)$ then $\alpha<\beta$
(D) If $a>0$ then $f(x)$ is decreasing on $(\alpha, \beta)$, where $\beta>\alpha$