The number of $x \in [0, 2 \pi]$ for which $| \sqrt{2 \sin ^4 x +18 \cos ^2 x} - \sqrt {2 \cos ^4 x +18 \sin ^2 x}|=1$ is :

Question

$\begin{array}{1 1}(1) 2 \\ (2)4 \\ (3) 6 \\ (4) 8 \end{array}$