  The ABC conjecture of Masser and Oesterle' states that if (a,b,c) are coprime
integers with a + b + c = 0, then sup(|a|,|b|,|c|) < c_e (rad(abc))^{1+e} for
any e > 0. Oesterle' has observed that if the ABC conjecture holds for all
(a,b,c) with 16 | abc, then the full ABC conjecture holds. We extend that
result to show that, for every integer N, the "congruence ABC conjecture" that
ABC holds for all (a,b,c) with N|abc implies the full ABC conjecture.
