The Acme Toy Company offers to sell Costco 10,000 dolls at a certain price. The offer, which is signed by Acme's President, is and certain as to all material terms. The offer also states that it will remain open for four months, but does not require any consideration in support of this promise. Acme revokes the offer 100 days after it was made. Is this revocation effective? Would your answer change if Acme revoked 80 days after the offer was made?