sanya, I guess the other answers are possible, but I think "set off" has a different meaning in your sentence. set off = located in a place apart by itself = alone It was an unusual (aviation) unit, located in a place apart from the others by itself, flying an easily identifiable aircraft. unit = group in the Air force, examples, -The mountain was set off by itself, remote and cold in the distance. -This truly private ground floor room with a dark cappuccino decor is set off by itself in the Carriage House, found by stepping through a separate garden. -A cozy pool room set off by itself is a perfect place for a group of your friends to play

Set off in that sentence means to "take off" like a plane takes flight.

set off machine = trigger a machine to start Usually its a trigger mechanism that sets off something. If you smoke in a public building it might set off the smoke alarm. If a machine sets itself off, it means it has an automatic trigger mechanism.