According to a study, after a week of high-altitude living, twenty men had slimmed down. The men, middle-aged residents of low-altitude areas, had been taken to a research station at 2, 650 meters (8, 694 feet) above sea level. They had unrestricted access to food and were forbidden vigorous exercise, yet they lost an average of 1. 5 kilograms (3. 3 pounds) during their one-week stay. Clearly, the lower availability of oxygen at higher altitudes, or hypobaric hypoxia, can be said to have caused the weight loss, since __. Which of the following would, if true, most logically complete the argument? ['exercise at higher altitudes is more difficult than exercise at lower altitudes is', 'a decrease in oxygen intake has been shown to depress appetite', 'the men all participated in the same kinds of exercise during their stay', 'several weeks after returning home, the men still weighed less than they had before the study']