Walter owes Christine $100. Walter does not have the money and does not think he will anytime soon. After discussing the matter, Christine agrees to take $50 as a full repayment. The parties sign a written agreement and Walter pays Christine $50. Disregarding promissory estoppel, can Christine sue Walter to collect the remaining $50?

a) Yes, because Christine's new agreement is considered a gratuitous promise and therefore not binding

b) No, because Walter completely performed his obligations under the new contract

c) No, because any new contract takes priority over old contracts

d) Yes, because Walter provided consideration for the new agreement