The stimulus tells that office workers reporting the least control over their own work had almost three times the risk of developing carpal tunnel syndrome, caused by repetitive motions, as did those who reported the most control. How can we explain the study's findings?
(B) tells us that workers who have the least control tend to experience emotional stress them more susceptible to nerve disorders. Since carpal tunnel is a nerve disorder, we can reasonably conclude that workers that have less control and more stress are more likely to develop carpal tunnel syndrome.
(D) is incorrect because it adds no additional information to explain the findings of the study but simply reiterates the conclusion of the study. The study conclude that among workers "who do similar amounts of typing," whereas (D) tells us that "among office workers who rarely use keyboard," i.e. workers who do similarly insignificant amount of typing, the rate of carpal tunnel is higher for those who lack control - which is essentially the same finding as in the study. The correct answer choice needs to explain the relationship between one's amount of control over his/her work and the likelihood of developing the syndrome.
Let me know if this makes sense and if you have any further questions.