【单选题】听力原文: Through a large amount of research, many American doctors have drawn the conclusion that mothers who smoke cigarettes before their babies are born may slow the growth of their babies' lungs. The...
A.
Examine whether these children were healthy.
B.
Find out whether their mothers had smoked.
C.
Find why these children suffered breathing problems and lung disease.
D.
Look into the effect that mother's smoking had on their children.