Let $f_K(x) = \frac{1}{k} (\sin^k x + \cos^k x)$ where $x \in R$ and $k \geq 1$. Then $f_4(x) - f_6(x)$ equals

Question

( A ) $\frac{1}{12}$
( B ) $\frac{1}{3}$
( C ) $\frac{1}{4}$
( D ) $\frac{1}{6}$