题目内容：
Having no language, infants cannot be told what they need to learn. Yet by the age of three they will have mastered the basic structure of their native language and will be well on their way to communicative competence. Acquiring their language is a most impressive intellectual feat. Studies of how children learn language generally agree that the most remarkable aspect of this feat is the rapid acquisition of grammar. Nevertheless, the ability of children to conform to grammatical rules is only slightly more wonderful than their ability to learn words. It has been reckoned that the average high school graduate in the United States has a reading vocabulary of 80,000 words, which includes idiomatic expressions and proper names of people and places. This vocabulary must have been learned over a period of 16 years. From the figures,it can be calculated that the average child learns at a rate of about 3 new words per day. Clearly a learning process of great complexity goes on at a rapid rate in children. 根据以上内容,回答题.
What is the main subject of the passage? A．Language acquisition in children.
B．Teaching languages to children.
C．How to memorize words.
D．Communicating with infants.
参考答案：
答案解析：