A homeowner hired a contractor to paint the homeowner's residence. The written contract stated that it was the parties' final and complete agreement and that all prior agreements between the parties merged into the written document. Prior to executing the contract, the contractor noted debris in the gutters of the residence. The contractor stated that to prevent such debris from adversely affecting the painting, the gutters should be cleaned. The contractor offered to do this prior to undertaking the painting for $600. The homeowner orally agreed. The homeowner and the contractor then signed the written contract, which did not mention cleaning the gutters. The contractor performed all of the work called for in the written contract as well as cleaning the gutters. The homeowner paid the amount specified in the written contract, but refused to pay an additional $600 for the cleaning of the gutters. In a breach of contract action by the contractor against the homeowner to recover the $600 payment, which of the following is the strongest argument that the homeowner can make to prevent the contractor from recovering?
a. The agreement regarding cleaning the gutters only serves to supplement the terms of the written contract.
b. Since the amount sought for cleaning the gutters was more than $500, it can only be evidenced by a writing.
c. The contract was a complete integration of the agreement between the contractor and the homeowner.
d. The parol evidence rule bars evidence about an oral agreement between the parties to a written contract.