$($contract$)$ State whether the statement is true, false, uncertain. If your answer is $$yes$,$ there is no need for explanation. Otherwise, explain in no more than $30$ words, or simply correct the statement.

a$.$In contract law, default rules are used to fill in gaps in contracts or to replace explicit terms in contracts that are in conflict with the default rules.$$

b$.$According to the bargain theory, a promise is not legally enforceable unless the size of consideration given by the other party is commensurate with the promise.$$