If the maximum and minimum values of $\sin ^ { 6 } x + \cos ^ { 6 } x$, as $x$ takes all real values, are $a$ and $b$, respectively, then $a - b$ equals\\ (A) $\frac { 1 } { 2 }$.\\ (B) $\frac { 2 } { 3 }$.\\ (C) $\frac { 3 } { 4 }$.\\ (D) 1 .