Gary owns two all-terrain vehicles (ATVs), worth $1,000 and $500, respectively. Helen enters into a contract to buy "Gary's ATV" for $750. Gary believes, in good faith, that he is selling the $500 ATV. Helen believes, in good faith, that she is buying the $1,000 ATV. In this situation: a) Gary is entitled to $750 for the $500 ATV. b) Helen is entitled to the $1,000 ATV for $750. c) Helen must buy both ATVs for $1,500. d) the contract is not enforceable.