A、By six months of age. B、By the end of their first year. C、By 18 months of age. D、By two years of age. A

Today, we will explore how intelligence develops in babies. We look at when and how babies learn, and the importance of early contact with languages.
   Do babies begin learning even before birth? Not long ago, people believed that babies only wanted food and to be kept warm and dry, and they were unable to learn things until five or six months old. (19)But the latest research finds that it is not the case. Babies are strongly influenced by the environment and the ability to learn exists even before birth. Newborn babies can recognize and understand sounds they heard while they were still developing inside their mothers.
   The first three years of a child’s life are the most intensive period of language and speech development. Language and communication skills are believed to develop best in an environment rich in sounds and sights. The first signs of communication happen during the first few days of life, when a baby learns that crying will bring food and attention. Research shows that (20) most children recognize the general sounds of their native language by six months of age. By that time, a baby usually begins to make sounds. These sounds become a kind of nonsense speech over time.
   By the end of their first year, most children are able to say a few simple words, although they may not understand the meaning. By 18 months of age, most children can say about 10 words. By two years of age, most children are able to make simple statements. By age three to five, the number of words a child can understand increases quickly. It is at these ages that children begin to understand the rules of language.
   Some children grow up in homes where more than one language is spoken. Understanding two languages can help children as they grow older. The youngest monolingual babies were able to recognize a difference between a language used at home and another language. But by 10 to 12 months of age, the monolingual babies were not identifying the sounds of the second language. By comparison, the bilingual babies did not differentiate sounds of different languages spoken to them between the ages of six and nine months. (21)But between 10 and 12 months, they could identify the different sounds of both languages.
   Questions 19 to 21 are based on the recording you have just heard.
   19. What do we know about the development of intelligence in babies according to the latest research?
   20. When do most children recognize the general sounds of their native language?
   21. What can the bilingual babies do by 10 to 12 months of age?