Carpal tunnel syndrome, a nerve disorder that affects the hands and wrists, is often caused by repetitive motions such as typing on a keyboard. A recent study of office workers found that, among those who do similar amounts of typing, workers reporting the least control over their own work had almost three times the risk of developing carpal tunnel syndrome as did those who reported the most control. Which one of the following, if true, most helps to explain the study's findings? ['Among office workers who rarely use keyboards, the rate of carpal tunnel syndrome is much higher for those who feel that they lack control over their own work.', 'Office workers who have the most control over their own work tend to perform repetitive motions other than typing more often than do office workers with the least control over their own work.', "The keyboards on which office workers type tend to put typists' arms and hands in positions that promote the development of carpal tunnel syndrome.", "Feeling a lack of control over one's own work tends to put one under emotional stress that makes one more susceptible to nerve disorders."]