Even if there is no contract, a plaintiff may be able to convince a court to enforce a defendant's promise if s/he can show that the defendant made a promise knowing that the plaintiff would likely rely on it; the plaintiff did rely on the promise, and the only way to avoid injustice is to enforce the promise. This legal principle is called a) substantial performance. b) substantial estoppel. Oc) c) promissory estoppel. d) frustration of purpose. e) commercial impracticability