单项选择题

Friendly Advice: The main cause for boys and girls to start drinking or smoking is whether they have friends who do, a National Institute of Health study suggests. Researchers find that school children are likely to do the same things as their five closest friends do, and girls are more likely than boys to give in to pressure to drink. However, the parents’ role makes a difference: Children whose moms and dads talk and listen to them regularly are less likely to smoke and drink.
Crying for a Smoke: New moms are often annoyed by the cries of new-bore babies, but according to one study, there is something they can do about it: Give up smoking. Researchers at a Netherlands organization of scientific research questioned parents of 3,000 babies up to six months old and found that stomachache was three times as likely in small babies whose mothers smoked 15 to 30 cigarettes a day, either before or after their babies were born. No data yet on Dad’s smoking, but it’s a safe suggestion that he should put the cigarettes out, too.
Out of Shape: Even as the number of overweight children increases, many schools are making unhealthy habits worse by cutting back on sports classes. Physical education is meant to provide exercise and encourage lifetime fitness, but a recent study found that only 26% of high schools require at least three years of physical education. It’s worse in lower grades: California middle school students get only 25 minutes of physical activity a week.
"Crying for a Smoke" suggests there is a cause-and-effect relation between ______.