Find the Largest Open Intervals Where the Function Is Concave $f(x)=x^{4}-8 x^{2}$

Multiple Choice

Find the largest open intervals where the function is concave upward.
- $f(x) =x^{4}-8 x^{2}$ (exact values)

A) None
B) $\left(-\infty,-\frac{2}{\sqrt{3}}\right) ,\left(\frac{2}{\sqrt{3}}, \infty\right)$
C) $\left(\frac{2}{\sqrt{3}}, \infty\right)$
D) $\left(-\infty,-\frac{2}{\sqrt{3}}\right)$