Under a contract with Valley View Villas, a residential development, Weng begins grading a terraced hillside. Halfway through the project, Weng asks for $5,000 over the contract price, claiming an increase in the "cost of doing business." Valley View agrees. This agreement to pay more isa. unenforceable because Weng’s request modified the contract. b.enforceable. c.unenforceable because Weng’s performance was a preexisting duty. d.unenforceable because Valley View’s promise was illusory.