一项工程，$Mike$单独做需要$2$小时完成，$Tom$单独做需要$4$小时完成$$.$$ $Mike$和$Tom$合作$1$小时后$Mike$退出，剩下的工作由$Tom$单独完成，还需要小时．  It takes Mike $$2$$ hours to finish a task． It takes $$4$$ hours for Tom to finish the same task． Mike and Tom worked together on this task for one hour before Mike had to leave． How long will it take Tom to finish the rest of the task? 
选项为:A:$$1$$  B:$$2$$  C:$$3$$  D:$$4$$  请回答其中一个选项.