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 Multiply to get a common denominator
3. Simplify

Fill in the missing step in the proof.
ad times cb all over bd
ad plus cb all over bd
ad minus cb all over bd
ad plus cb all over two times bd

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