A buyer signs a purchase contract at the asking price with no contingencies and the $30-$day closing date as specified by the see. The seller changes the contract to reflect a closing date of $40$ days and signs the contract. This contract, in its presentform, isO A vall' with the $30$ day closing date, since the buyer met all the terms as originally specified by the seller.O B$.$ valid with the $40-$day closing date, since a $10-$day extension is not considered material to the contract.O C$.$ Invali, since the signed documents do not yet indicate mutually agreed$-$upon terms.O D$.$ enforceable, since the seller has provided a valid counter$-$offer.