How can courts prevent an offeror from revoking a unilateral contract once the offeree has started performance?

Question

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Courts applying modern contract rules may prevent an offeror from revoking a unilateral offer once the offeree has begun performance.This is achieved by holding either that a bilateral contract is created by the beginning of performance or that the offeror's power to revoke is suspended for the period of time reasonably necessary for the offeree to complete performance.

How Can Courts Prevent an Offeror from Revoking a Unilateral