In case of doubt an offer is interpreted as inviting the offeree to accept either by promising to perform what the offer requests or by rendering the performance, as the offeree chooses.

Comments:

a.....The offeror is often indifferent as to whether acceptance takes the form of words of promise or acts of performance, and his words literally referring to one are often intended and understood to refer to either. Where performance takes time, however, the beginning of performance may constitute a promise to complete it. See §62.

b. Offer limited to acceptance by performance only. Language or circumstances sometimes make it clear that the offeree is not to bind himself in advance of performance. His promise may be worthless to the offeror, or the circumstances may make it unreasonable for the offeror to expect a firm commitment from the offeree. In such cases, the offer does not invite a promissory acceptance, and a promise is ineffective as an acceptance. Examples are found in offers of reward or of prizes in a contest, made to a large number of people but to be accepted by only one.