The observed effect that the author is trying to explain? "Businesspeople who travel internationally on business are much more likely to suffer from chronic insomnia than are businesspeople who do not travel on business."
The author's proposed cause? "International travelers experience the stresses of dramatic changes in climate, frequent disruption of daily routines, and immersion in cultures other than their own . . . "
This is a Strengthen question so we are looking for an answer choice that will strengthen the author's argument.
(C) strengthens this argument by establishing that these two groups (i.e. international travelers and non-international travelers) were the same prior to accepting assignments that require international travel.
Notice that this answer choice rules out the possibility that people with chronic insomnia being more likely to accept positions that require international travel than those who do not suffer from such insomnia.
Hope that helps! Please let us know if you have any other questions.
