Answer:
B) Is not a contract because there is no consideration for B's promise.
Step-by-step explanation:
In contract law, consideration 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 B added some new consideration (more money) to the written contract, A didn't add anything new.