A、Humorous individuals. B、People who had undergone treatment for blocked arteries. C、People who did not laugh much. D、People who

Laughing at life is one of the best ways of guaranteeing it does not end prematurely, scientists said yesterday. Researchers found that people who often laugh out loud and see the funny side of difficult situations are far less likely to have a heart attack than humorless individuals.
The study looked at the humor responses of 150 patients who had either suffered heart attacks or undergone treatment for blocked arteries. They were compared with the responses of an identical number of age-matched healthy people who had no history of heart problems. There was some variation in the results, but, overall, it was found that the heart patients were 40% less likely than their healthy counterparts to laugh in a variety of common situations.
Michael Miller, director of the Center for Preventive Cardiology at the University of Maryland, Baltimore, who led the research, said: "The old saying that laughter is the best medicine appears to be true when it comes to protecting your heart."
The findings suggested that making oneself laugh when angry might offset the harmful effect of stress on arteries. The team’s findings were presented yesterday at the American Heart Association’s annual scientific meeting in New Orleans.
What is the composition of the sample that the research focuses on?

According to the research，what is the characteristic of people who are most likely to have a heart attack?