Park$-$Lot Maintenance Corp. entered into a three$-$year contract with Beliis Fair Mall for snow removal. The contract was in writing, but it was agreed orally that the snow would need to be at least $10$ centimetres deep before the contract was operative. Which is the applicable legal concept?

a$.$

The contract is in effect regardless of the oral stipulation.

b$.$

The parol evidence rule will not allow the oral stipulation to be enforced by the courts

c$.$

The oral part is a condition precedent and therefore renders the contract void.

d$.$

The oral part is a condition subsequent and is effective for three years

e$.$

The oral part is a condition precedent and therefore enforceable