Mary's dog Spot is lost, so she offers a $100 reward for anyone who finds Spot and brings him home. The town dog catcher finds Spot, brings him home to Mary and demands the reward.

a. Mary must pay because this was a bilateral contract and the act required as consideration was completed.

b. Mary must pay because she promised to do so.

c. Mary need not pay because the dog catcher was obligated to catch dogs, and his pre-existing duty to catch dogs like Spot could not be consideration for the unilateral contract here.

d. Mary need not pay because her offer was made when she was under duress.