John as well as Mike __________ just been back from an important meeting.A. haveB. hasC. had

此题考察主谓一致。当主语后跟有as well as, together with, along with, besides, except, but, like, including, in addition to等连接的词组时，谓语动词应与主语保持一致。因此，主语John为单数，谓语动词应使用单数形式。根据句意，John和Mike刚刚从一个重要的会议回来，所以使用现在完成时，即has just been back。