Katherine mailed Paul an offer with definite and certain terms and that was legal in all respects stating that it was good for 10 days. Three days later, Paul receives the offer and agrees to it by signing the documents and Paul mails back the acceptance by certified mail (time and date stamped by the Postal service). That evening Paul calls Katherine to inform her about his acceptance, she replies back saying that she would like to revoke her offer. Paul answers back that she cannot revoke the offer and that they have a legally binding agreement.

is the revocation made by Katherine effective?