Mark, a newspaper editor, walks into the newsroom and announces to a group of five reporters: "I'll pay a $2,000 bonus to the first reporter who finds definitive evidence that Senator Blue smoked marijuana at the celebrity party last Friday." Anna, the first reporter to produce the evidence, claims her bonus based on:
(a) Unilateral contract
(b) Promissory estoppel
(c) Quasi-contract
(d) Implied contract
(e) Express contract