Erin Receives an Offer in the Mail from Susan on July

Erin receives an offer in the mail from Susan on July 14 that says,"I will sell you my vacation home for $80,000.You may accept this offer by signing the bottom of this letter and placing it in the mail so that it is postmarked by July 31." Which is true?

A) Susan will be liable for breach of contract if she sells that home to anyone else prior to July 31.
B) If Erin accepts the offer on July 20 by meeting the terms of the offer,she (Erin) can change her mind up until July 31 so long as Susan is notified by that date.
C) Assuming Erin has not paid Susan for an option contract,Susan may revoke the offer at any time so long as she does so before Erin accepts.
D) If Erin counteroffers with a price of $75,000,and Susan rejects,Erin can create a contract by accepting the original offer by July 31.