How to Ace FYLSE/2010 F Q1

https://admissions.calbar.ca.gov/Portals/4/documents/gbx/sf_1002_Essay-Selected_Answers.pdf

Fact Summary
P entered into a contract with D to write computer programs in which D was to pay P $25,000 on completion and to deliver them by May 1. The contract stated "This is the complete and entire contract between the parties, and no modification of this contract shall be valid unless it is in writing and signed by both parties."