Which of the following best describes a firm offer: a) Either party can revoke at any time. b) It is revocable for lack of consideration during the stated time or, if no time is stated, for a reasonable time not to exceed six months. c) It is an offer given to a merchant to purchase a firm quantity of goods for a period of up to six months. d) It is an offer made by a merchant not revocable for lack of consideration during the stated time or, if no time is stated, for a reasonable time not to exceed three months