Where a declaration of will to revoke an offer is made by means of dialog, the contents of the declaration of will shall come to the offeree's knowledge before the offeree accepts; and a declaration of will to revoke an offer made by means other than dialog shall reach the offeree before the offeree accepts. 撤销要约的意思表示以对话方式作出的，该意思表示的内容应当在受要约人作出承诺之前为受要约人所知道；撤销要约的意思表示以非对话方式作出的，应当在受要约人作出承诺之前到达受要约人。 An offer lapses under any of the following circumstances: (1) The offer is rejected. (2) The offer is legally revoked. (3) The offeree fails to accept before the expiration of the time limit for acceptance. (4) The offeree makes a substantial modification to the contents of the offer. 有下列情形之一的，要约失效： （一）要约被拒绝； （二）要约被依法撤销； （三）承诺期限届满，受要约人未作出承诺； （四）受要约人对要约的内容作出实质性变更。