Read the text and answer question.
Meet the 'homeless man' who flies the world first class
[1] For the last year and a half, Ben Schlappig has been
essentially homeless. The 25-year-old American has ______
fixed address, ______ the keys to his own front door. Why
bother paying rent, he figured, when a bed was only ever a
[5] first class ticket away?
Schlappig is a "Hobbyist," as those in the game call it - a
professional traveler with an almost obsessive attention to
fine print, who uses a mixture of frequent flyer miles and
credit card reward points, to zoom around the globe for a
[10] fraction of the cost.
Since he first started racking up air miles a little over a
decade ago – yes, he was just 14 when he started this –
Schlappig says he's never flown economy on an international
flight.
Taken from http://edition.cnn.com/2015/12/01/travel/ben-schlappig-flyworld-free-airline-air-miles/index.html
Choose the correct verb that can replace the phrasal verb “racking up” (line 11), according to the text.
