A contractor refuses to complete a house for $$150,000$ because he has already spent that

amount in building the house. He demands an additional $$50,000$ from the owner to complete

the house even though the contract calls for a completed house for $$150,000.$ If the owner agrees

to his demand

a$.$ This additional promise is unenforceable because of pre existing legal duty

b$.$ This additional promise is unenforceable because of past consideration

c$.$ This additional promise is enforceable