ACCA f4的一题题目,应该不是很难,请大家说说思路

ACCA f4的一题题目,应该不是很难,请大家说说思路
Peter advertised his car for sale in a local newspaper and then went on holiday leaving his car in the drive. While Peter was away, Tom, having seen the advertisement, went to look at the car and decided to make an offer for it. Peter’s neighbour, Alf, pretending to act with Peter’s authority, entered into negotiations with Tom and eventually accepted Tom’s offer on Peter’s behalf. Alf had no authority to act in this way. When Peter returned from his holiday, he wrote to Tom saying that he was ratifying Alf’s act.

Advise Tom as to whether he is bound by the contract.