Which of the following is correct regarding contracts that have been subsequently modified?

Multiple Choice

a. Oral evidence of a subsequent written agreement is inadmissible.

b. Parol evidence regarding a contract's subsequent modification is inadmissible.

c. If the agreement is required to be in writing because it is within the statute of frauds, oral modifications are unenforceable.

d. Parol evidence contradictory to the final terms is admissible.

e. If the contract's terms require that modification be in writing, oral modifications are admissible.