Farmer Smith Hires Joe to Drill a New Well

Farmer Smith hires Joe to drill a new well.Joe looks at the drilling site and quotes Smith a price of $500 for the new well.After drilling a short distance,Joe discovers hard bedrock,which is unexpected in this locality.To drill through this would take substantially more time and cost more.Joe says he will continue,but only if Smith pays him $2,000.Smith agrees.Smith's promise to pay an increased amount is enforceable because:

A) This is a valid settlement of a liquidated debt.
B) This is an unforeseen circumstance.
C) This is a Uniform Commercial Code modification.
D) This is a preexisting duty.
E) This is a novation.