Psychologist George Spilich and colleagues at Washington College in Chestertown, Maryland, decided to find out whether, as many smokers say, smoking helps them to "think and concentrate." Spilich put young non-smokers, active smokers and smokers deprived   of cigarettes through a series of tests. In the first test, each subject   sat before a computer screen and pressed a key as soon as he or she recognized a target letter among a grouping of 96. In this simple test, smokers, deprived smokers and nonsmokers performed equally well. The next test was more complex, requiring all to scan sequences of 20 identical letters and respond the instant one of the letters transformed into a different one. Non-smokers were faster, but under the stimulation of nicotine  , active smokers were faster than deprived smokers. In the third test of short-term memory, non-smokers made the fewest errors, but deprived smokers committed fewer errors than active smokers. The fourth test required people to read a passage, then answer questions about it. Non-smokers remembered 19 percent more of the most important information than active smokers, and deprived smokers bested those who had smoked a cigarette just before testing. Active smokers tended not only to have poorer memories but also had trouble separating important information from insignificant details. "As our tests became more complex." Sums up Spilich, "non-smokers performed better than smokers by wider and wider margins" He predicts, "smokers might perform adequately at many jobs until they got complicated. A smoking airline pilot could fly adequately if no problems arose, but if something went wrong, smoking might damage his mental capacity." The purpose of George Spilich's experiments is   _  .
to test whether smoking has a positive effect on the mental capacity of smokers to show how smoking damages people's mental capacity to prove that smoking affects people's regular performance to find out whether smoking helps people's short-term memory