请阅读Passage 2，完成第1—5小题。

Passage 2

  Is language,like food,a hasic human need without which a child at a critical period of life can he starved or damaged'? Judging from the experiment of Freclerick lI in the 13th century it may be. Hoping to discover what language a child would speak if he heard no mother tongue as he told the nurses to keep silenl.

  All the infants died before the first year. But clearly there was more than a mere problem of lacking language. What was missing was good mothering. Without good mothering,in the first year of life especially,the capacity to survive is seriously affected.

  Today no serious lacking exists as that ordered hy Frederick. Nevenhcless,some children are still backward in speaking. Most often the reason for this is that the mother is insensitive to the clues and signals of the infant,whose brain is programmed to pick up language rapidly. If these sensitive periods are neglected,the ideal time for acquiring skills passes and they might never be leamed so easily again. A bird learns to sing and to fly rapidly at right time,but the process is slow and hard once the critical stage has passed.

  Scientists suggest that speech milestones are reached in a fixed sequence and at a constant age,but there are cases,where speech has stafled late in a child who eventually turns out to be of high intelligencc. At twelve weeks a baby smiles and makes bowel-like sounds;at twelve months he can speak simple words and understand simple commands;aL eighteen months he has a vocabulary of three to fifty words. At three he knows about I ,000 words which he can pul into sentences,and at four his language differs from that of his parents inbom with the capacity to speak. What is special about man's brain,compared with that of the monkey,is the complex system which enables a child to comment the sight and feel of,say,a toy-bcar with the souncl pattern" toy-bear". And even more incredible is the young brain's ability to pick out an crder in language from the mixture of sound around him,to analyse,to combine and recombine the parts of a language in new ways.

  But speech has to be induced,and this depends on interaction between the mother and the child,where the mother recognizes the signals in the child's habbling,grasping and smiling,and respond8 to them. Insensitivity of the mother to thesc signals dulls the interaction because the child gets discouraged and sends out only the obvious signals. Sensitivity to the child's non-verbal signals is esqential to the growth and development of language.