Show that for every prime ( p ), ( (p - 1)! ≡ p - 1 mod p ).

a) ( (p - 1)! ≡ 1 mod p )
b) ( (p - 1)! ≡ 0 mod p )
c) ( (p - 1)! ≡ p mod p )
d) ( (p - 1)! ≡ -1 mod p )