If the buyer accepts, there is a contract for the quantity and type of goods actually sent by the seller, not for those originally ordered by the buyer. If the buyer rejects, he can send back the goods. In any event, seller will not be found to be in breach. Where an offer looks to a unilateral contract, most courts now hold that the offeree must give notice of his acceptance after he has done the requested act. If he does not, the contract that was formed by the act is discharged.
B crosses the bridge on time.
As soon as B crosses, a contract is formed.