[单项选择]For years, smokers have been exhorted to take the initiative and quit: use a nicotine patch, chew nicotine gum, take a prescription medication that can help, call a help line, just say no. But a new study finds that stopping is seldom an individual decision. Smokers tend to quit in groups, the study finds, which means smoking cessation programs should work best if they focus on groups rather than individuals. It also means that people may help many more than just themselves by quitting: quitting can have a ripple effect prompting an entire social network to break the habit.
The study, by Dr. Nicholas Christakis of Harvard Medical School and James Fowler of the University of California, San Diego, followed thousands of smokers and nonsmokers for 32 years, from 1971 until 2003, studying them as part of a large network of relatives, co-workers, neighbors, friends and friends of friends.
It was a time when the percentage of adult smokers in the United States fell to 21 percent
A. at large
B. all together
C. in the end
D. respectively

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