Martinson Entered into a Contract to Purchase a Used Fax

Martinson entered into a contract to purchase a used fax machine from Gruber for $550. Martinson, who did not sign the agreement, changed his mind and refused to buy the fax machine from Gruber. Can Gruber legally enforce the contract?

A) No; the sale was for less than $600.
B) No; Martinson did not sign the contract.
C) Yes; Gruber signed the contract.
D) Yes; the sale contained all the elements of a contract.