the emergency room

Business law teacher, Debby, needed some yard work done. She told her class that she would give $50 to the first person who mowed her yard. Max went to mow Debby’s yard. Unfortunately, just as he finished mowing, a neighborhood dog bit him, and he had to go to the emergency room for a couple of stitches. Debby refused to pay Max on the basis that the agreement was not in writing and that Max ended up being more trouble than he was worth. Max refused to pay the emergency room because he said that he did not have a binding, bilateral contract with Debby. Which of the following is an appropriate characterization of the agreement between Debby and Max?

They had a bilateral, express contract.
They had a bilateral, implied contract.
They had a bilateral and unilateral contract.
They had a unilateral contract.
They did not have any type of enforceable agreement.