National Trucking Company Begins Transporting Crude for Oil Inc

Multiple Choice

National Trucking Company begins transporting crude for Oil Inc. under a contract for a stated amount per load. After six months, National demands an increase in the stated amount because ordinary business expenses have increased. Oil's agreement to pay the increased amount is

A) enforceable as the consideration is past.
B) enforceable due to unforeseen difficulties.
C) unenforceable as an illusory promise.
D) unenforceable due to the preexisting duty rule.