Lecture 19 (Discrete Math) Number Theory, Modular Arithmetic, and Factoring

In Z sub n, what does b being a reciprocal of a mean?

1. A.a and b add together to give exactly n
2. B.a times b leaves a remainder of zero
3. C.a times b is congruent to 1 mod n
4. D.b is the unique additive inverse of a