In the following question, I do not understand why the answer is B. When can conditions be oral vs. written? Please explain clearly.

Question In a writing signed by both parties on December 1, a man agreed to buy from a distributor a gasoline engine for $1,000, delivery to be made on the following February 1. Through a secretarial error, the writing called for delivery on March 1, but neither party noticed the error until February 1. Before signing the agreement, the man and the distributor orally agreed that the contract of sale would be effective only if the man notified the distributor in writing no later than January 2 that the man had arranged to resell the engine to a third person. Otherwise, they agreed orally, "There is no deal." On December 15, the man entered into a contract with a mechanic to resell the engine to the mechanic at a profit. The man did not give the distributor notice of the resale until January 25, and the distributor received it by mail on January 26. In the meantime, the value of the engine had unexpectedly increased about 75% since December 1, and the distributor renounced the agreement. If the man sues distributor on February 2 for breach of contract, which of the following is the distributor's best defense? ( suing it forunlatmstice The secretarial error'in the written delivery-term was a mutual mistake concerning a basic fact, and the agreement is voidable by either party. B The man's not giving written notice by January 2 of his resale was a failure of a condition precedent to the existence of a contract. In view of the unexpected 75% increase in value of the engine after December 1, the distributor's performance is excused by the doctrine of commercial frustration. D The agreement, if any, is unenforceable because a material term was not included in the writing.Makal tem wes increed bit + was wing