If $f_k(x) = \large\frac{1}{k}$$(\sin^k x + \cos^k x)$, where $x\in R$ and $k \geq 1$. The $f_4(x) - f_6(x)$ equals

Question

$\begin{array}{1 1} \frac{1}{6} \\\frac{1}{3} \\ \frac{1}{4} \\ \frac{1}{12} \end{array}$