Ayoga instructor entered into a valid written contract with a builder to construct a large yoga studio on some land she owned outside of town. she agreed to pay the builder $150,000 upon completion of the job. as work progressed, and due to substantial increased building costs, the yoga instructor and the builder orally agreed that the builder may omit installation of the koi pond planned for the atrium (saving the builder $1,000), and that the contract price would be reduced to $149,500. the builder completed the job (minus the koi pond) in reliance thereon. most courts would hold that this subsequent oral agreement is:
a. an enforceable contract.
b. unenforceable under the statute of frauds.
c. unenforceable, because a contract in writing cannot be modified orally.
d. unenforceable under the parol evidence rule.