A、By smiling to please others when they need care. B、By repeating one behavior familiar to their mothers’. C、By changing the env

(16)Not long ago, many people believed that babies only wanted food and to be kept warm and dry. Some people thought babies were not able to learn things until they were five or six months old. Yet doctors in the United States say babies begin learning on their first day of life. Research scientists note that babies are strongly influenced by their environment. They say a baby will smile if her mother does something the baby likes.(17)A baby learns to get the best care possible by smiling to please her mother or other caregiver. This is how babies learn to connect and communicate with other people. The American researchers say this ability to learn exists in a baby even before birth. They say newborn babies can recognize and understand sounds they heard while they were still developing inside their mothers.(18)A new study in California says the first three years of a child’s life are important for all later development. Experts from the study say mothers can strongly influence social development and language skills in their children in these three years.(19)Another study suggests that low birth weight babies with no evidence of disability may be more likely than other children to have physical and mental problems. The study found that the young people with low birth weight often had more problems with movement skills than others. These problems were more common among males, those with injured nerve tissue in the brain and those who had been given oxygen supplies for days as a baby.
16. What did people think of babies in the past?
17. How do babies learn to communicate with others?
18. What does the study in California show?
19. What does the speaker say about underweight babies?