Which of the following is true about the exception to the parol evidence rule?

Select one:

a. A party can always introduce proof of an oral agreement made before the writing was created.

b. Oral testimony that attempts to show that the contract is illegal is not allowed.

c. A party is allowed to use oral terms to alter, vary, or contradict the written terms of the contract.

d. If the terms of the writing are unclear, oral testimony can be introduced to aid the court in interpreting the writing.