John and Jack have been studying English for several hours.
Who English for several hours? (John and Jack)
What they for several years? (English)

Harry has been playing tennis for several years.
Who tennis for several years? (Harry)
What he for several years? (tennis)
What he for several years? (playing tennis)

They have been travelling around the world for six weeks.
What they for six weeks? (travelling around the world)
Who around the world for six weeks? (They)