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? 0 If one person promises to pay more for completion ofa contract, the promise may be gratuitous. O Gratuitous promises mean that there is an exchange of consideration. 0 A public duty may be a gratuitous promise. 0 If one person has an existing obligation to the other person in a contract, a subsequent agreement is illegal. 0 Modications to an existing agreement require only the parties' agreement