Agreements to complete an existing legal obligation under a contract have a problem with consideration, Which of the following is the best reason for this rule? O If one person promises to pay more for completion or a contract, the promise may be gratuitous, If one person has an existing obligation to the OthefperSon in a contract, a subsequent agreement is illegal. A public duty may be a gratuitous promise. Gratuitous promises mean that there is an exchange Ofconsideration. Modifications to an existing agreement require only the parties' agreement.