Buyer Jon signs a purchase contract at the asking price with no contingencies and the 30 day closing date as specified by the seller. The seller changes the contract to reflect a closing day of 40 days and signs the contract. This contract in its present form is:

a Valid with the 30-day closing date, since Buyer Jim ...the terms as originally specified by the seller

b Valid with the 40-day closing date, since a 10-day extension is not considered material to the contract

c Invalid since the signed documents do not yet indicate mutually agreed upon terms

d Enforceable, since the seller has provided a valid counter offer