Which of the following offerors is free to revoke his offer?

An offeror of a unilateral contract where the offeree already started performance.

An offeror, who is a merchant, agreed in writing to sell goods and gave assurances that the offer would remain open.

An offeror that agreed to keep an offer open without consideration.

An offeror who promised to keep an offer open without consideration, but later changed his mind and the offeree foreseeably and reasonable relied on the offeror's promise.

Yasmin offers to landscape Bert's property for $2,000. Two days after making the offer, Yasmin changes her mind and mails Bert a letter revoking her offer. The next day, Bert, who has not received Yasmin's letter, calls Yasmin and accepts her offer. Does a contract exist?

Yes, because mail is not a proper method of revoking an offer.

No, because Yasmin can revoke her offer at any time.

No, because Yasmin mailed her revocation before Bert's acceptance.

Yes, because Bert was not notified of Yasmin's revocation at the time he accepted.

A promisee's promise not to commit a crime can never be consideration because:

the promise is not bargained for and given in exchange.

the promise is illusory.

every member of society has a duty to obey the law.

it lacks future intent.