Which of the following would be considered a valid and legally enforceable agreement? a. An agreement to pay a legislator to vote for a particular bill b. An agreement under which Arthur agrees to pay Barbara $3,000 to disparage the product of Arthur's competitor c. An agreement by the seller of a service station not to enter the service station business in Ohio for a period of 30 years after the sale of his business d. An agreement by a car salesman not to sell automobiles or automobile parts in Marin County for a period of one year after terminating his employment