Rosita paid John $1, 500 to paint her apartment. After John completed the job. she promised him $100 more for his excellent work. Is this additional promise of $100 an enforceable contract? a) No, because it is supported only by a preexisting obligation. b) Yes, because it is supported by valid consideration. c) No, because it is supported only by past consideration. d) No, because it involves an unliquidated debt