Prove the sum of two rational numbers is rational where a, b, c, and d are integers and b and d cannot be zero.

steps reasons
1. a over b plus c over d given
2. ad over bd plus cb over bd
3. ad plus cb all over bd simplify

fill in the missing reason in the proof.
a: multiply to get a common denominator.
b: add to get a common denominator.
c: distribute d to all terms.
d: add d to all terms.