单项选择题

Is language, like food, a basic human need without which a child at a critical period of life can be starved and damaged Judging from the experiment of Frederick Ⅱ in the thirteenth century, it may be. Hoping to discover what language a child would speak if he heard no mother tongue, he told the nurses to keep silent.
All the babies died before the first year. But clearly there was more than lack of language here. What was missing was good mothering. Without good mothering, in the first year of life especially, the ability to survive is seriously influenced.
Today no such severe lack exists as that ordered by Frederick Ⅱ Nevertheless, some children are still backward in speaking. Most often the reason for this is that the mother is insensitive (不敏感的) to the signals (信号) of the baby, whose brain is made to learn language rapidly. If these sensitive periods are neglected, the ideal time for acquiring skills passes and they might never be learned so easily again. A bird learns to sing and to fly rapidly at the right time, but the process is slow and hard once the critical stage has passed.
Experts suggest that speech stages are reached in a fixed order and at a constant age, but there are cases where speech has started late in a child who eventually turns out to be clever.
At twelve weeks a baby smiles and makes vowel-like sounds, at twelve months he can speak simple words and understand simple commands, at eighteen months he has a vocabulary of three to fifty words. At three he knows about 1000 words which he can put into sentences, and at four he knows his language differs from that of his parents in style rather than grammar. A child can say "Mum" probably at ______.