Answer:
The answer is: B) there is no consideration for Jack's promise.
Step-by-step explanation:
Consideration (in contract law) is the benefit that must be bargained for between the parties involved. It is the essential reason for the parties entering a contact. Consideration must have some value and is exchanged on the performance or promise from the other party.
Common law rules on contract modifications require some new consideration in order to modify an existing contract. In this case, only Jack added some new consideration (more money) to the written contract, Joe didn't add anything new.