Onsite Restoration Inc. begins renovating houses for Property Company under a contract for a stated amount per house. After six months, Onsite demands an extra $20,000 per house, stating no reason for the extra $20,000, but asserting that it will stop work if it is not paid. The agreement is a. unenforceable due to the preexisting duty rule. b. unenforceable as an illusory promise. c. enforceable due to unforeseen difficulties. d. enforceable as the consideration is past