Let $F$ be a field with 256 elements, and $f \in F [ x ]$ a polynomial with all its roots in $F$. Then,\\
(a) $f \neq x ^ { 15 } - 1$;\\
(b) $f \neq x ^ { 63 } - 1$;\\
(c) $f \neq x ^ { 2 } + x + 1$;\\
(d) if $f$ has no multiple roots, then $f$ is a factor of $x ^ { 256 } - x$.