when can an offer to form a unilateral contract be revoked
An offeror may revoke an offer before it has been accepted, but the revocation must be communicated to the offeree (although not necessarily by the offeror,). If the offer was made to the entire world, such as in Carlill's case, the revocation must take a form that is similar to the offer. However, an offer may not be revoked if it has been encapsulated in an option (see also option contract), or if it is a "firm offer" in which case it is irrevocable for the period specified by the offeror.