题目内容：
根据以下材料，回答题Why Don't Babies Talk Like Adults?
Over the past half-century, scientists have settled on two reasonable theories related to babytalk. One states that a young child's brain needs time to master language, in the same way that it does to master other abilities such as physical movement. The second theory states that a child's vocabulary level is the key factor. According to this theory, some key steps have to occur in a logical sequence before sentence formation occurs. Children's mathematical knowledge develops in the same way.
In 2007, researchers at Harvard University, who were studying the two theories, found a clever way to test them. More than 20, 000 internationally adopted children enter the U. S. each year. Many of them no longer hear their birth language after they arrive, and they must learn English more or less the same way infants do--that is, by listening and by trial and error. International adoptees don't take classes or use a dictionary when they are learning their new tongue and most of them don't have a well-developed first language. All of these factors make them an ideal population in which to test these competing hypotheses about how language is learned.
Neuroscientists Jesse Snedeker, Joy Geren and Carissa Shafto studied the language development of 27 children adopted from China between the ages of two and five years. These children began learning English at an older age than US natives and had more mature brains with which to tackle the task. Even so, just as with American-born infants, their first English sentences consisted of single words and were largely bereft (缺乏的) of function words, word endings and verbs. The adoptees then went through the same stages as typical American-born children, though at a faster clip. The adoptees and native children started combining words in sentences when their vocabulary reached the same sizes, further suggesting that what matters is not how old you are or how mature your brain is, but the number of words you know.
This finding--that having more mature brains did not help the adoptees avoid the toddler-talk stage-- suggests that babies speak in babytalk not because they have baby brains, but because they have only just started learning and need time to gain enough vocabulary to be able to expand their conversations. Before long, the one-word stage will give way to the two-word stage and so on.
Learning how to chat like an adult is a gradual process.
But this potential answer also raises an even older and more difficult question. Adult immigrants who learn a second language, rarely achieve the same proficiency in a foreign language as the average child raised as a native speaker. Researchers have long suspected there is a "critical period" for language development, after which it cannot proceed with full success to fluency. Yet we still do not understand this critical period or know why it ends.
What is the writer's main purpose in Paragraph 2? A．To reject the view that adopted children need two languages.
B．To argue that culture affects the way children learn a language.
C．To give reasons why adopted children were used in the study.
D．To justify a particular approach to language learning.
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