[单项选择]

American doctors say mothers who smoke cigarettes may slow the growth of their children’ s lungs (肺). They said reduced lung growth could cause the children to suffer breathing problems and lung diseases (疾病) later in life. Doctors studied more than 1 100 children between the ages of five and nine. The mothers of some of the children smoked; the other mothers did not. Doctors tested the children once a year for five years to see how fast their lungs were growing. The test measured the amount of air the children could blow out of their lungs in one second. Children should be able to blow out more air each year because their lung-power increases as their lungs develop. But the doctors found that the lungs of the children whose mothers smoked had not developed as fast as they should. Doctors are not sure when the mothers’ smoking affected (影响) the children’s lungs. They say it could have happened before birth because the mothers smoked during pregnancy (怀孕) or it c
A. Ten times.
B. Five times.
C. Once.
D. Twice.