[单项选择]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." Spihch put young non-smokers, active smokers and smokers deprived (被剥夺) of cigarettes through a series of tests. In the first test, each subject (试验对象) 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 (尼古丁), 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 fourt
A. to test whether smoking has a positive effect on the mental capacity of smokers
B. to show how smoking damages people’s mental capacity
C. to prove that smoking affects people’s regular performance
D. to find out whether smoking helps people’s short-term memory

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