A offers to sell B a quantity of ball bearings, using a regular letter to make the offer. This is the customary way of communicating offers and acceptances in the ball bearing trade. However, B decides to accept by carrier pigeon--a most unreliable means of acceptance. This time, though, the carrier pigeon delivers the acceptance to A in exactly the same time that it would have taken a letter to arrive. But after the pigeon took off, but before it reached A, A telephoned B to revoke his offer. Is there a contract between A and B?