单项选择题

The strange, close understanding between twins is a familiar enough phenomenon. Often they seem to understand each other and share each other’s emotions to such an extent that one suspects some kinds of thought communication.
What is not so widely known is that this special relationship often acts as a brake (障碍) on twins’ intellectual development. As they are partly isolated in their own private world, twins communicate less with adults than do other children. The verbal (口头的) ability of a four-year-old twin is typically six months behind that of a non-twin. The problem can be particularly severe in a deprived home, a one-parent family for example, where there is little stimulation for children anyway.
Such children, while capable of mutual comprehension in a private language, often remain incomprehensible to outsiders and thus at a severe educational disadvantage. The only solution to the problem, cruel though it may seem, is to separate the twins, thus forcing them to acquire ordinary speech helped and guided by sympathetic (富有同情心的) parents and teachers.

The writer mentions all of the items listed below EXCEPT（）

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