Reading Comprehension 
从下列每篇短文的问题后所给的四个选择项中选出一个最佳答案。
     When children learn a language, they learn the grammar as well as the words or vocabulary. No one teaches them, children just "pick them up". Before babies begin to produce words, they produce sounds. Some of these sounds will remain if they occur in the language being acquired, and others will disappear. This is called the "babbling（牙牙学语）stage". 

     A child does not learn the language "all at once". The child's first utterances（言语）are one-word sentences. After a few months, the two-word stage arises. During this stage, the child puts two words together. These two-word sentences have definite patterns and express grammatical and meaningful relationships.  
     Still later, in the telegraphic stage, the child will produce longer sentences. These longer sentences are mainly made up of content words. The child's early grammar lacks many of the rules of the adult grammar, but eventually it will become mature.
     All normal children everywhere learn language. This ability is not dependent on race（种族），social class, geography, or even intelligence. This ability is uniquely human.