Could you explain which option is correct?
By the time you get to the airport I... for two hours.
Will rest
Will have been resting
Will be resting

Question

Could you explain which option is correct? 
By the time you get to the airport I... for two hours. 
Will rest
Will have been resting 
Will be resting

Answer

Will have been resting.

Is the best

Answer

Because we are focusing on an activity that hasn’t finished.

Hence the use of present perfect continuous.