单选题 .  Learning is an essential process for living things to acquire necessary skills and behaviors. Scientists have already found that there is a critical period for learning in man and animals. Once the right time for learning a skill or a certain behaviors has passed, this skill or behavior can never be properly learned.
    That we speak our own language perfectly and the languages we learn later imperfectly also depends on learning this skill at the right time. The brain develops in such a way that it is ready to learn to speak between the ages of about ten months and ten years. Languages learned later are not spoken perfectly. The ability to speak a language is a mixed motor and sensory skill. It is necessary to hear all the sounds, the subtle differences between similar but not identical sounds, the rhythm and tone of the language. One has also to work tongue, throat, and lips to control breathing. All this has to be managed at one and the same time. From some time after birth until the age of six or seven, normal children all learn this perfectly, and without much difficulty they can learn two or even three languages at the same time, without confusing one with the other. But later, most of us cannot acquire this skill. We may learn to write and read the new language perfectly, but to acquire the right accent and the ability to speak so that no one can detect that the language is not our mother tongue hardly ever occurs.
    But those of us who still want to learn foreign languages after these early milestones have been passed need not worry, for though we may not learn to speak a new language like the natives, we know from thousands of examples that we can go on learning languages beyond the age of eighty. There are so many activities adding up to the simple word "learning" that although some of the processes become less efficient with ageing of the brain, our actual experience of learning helps us in learning new materials.
    Normal social and emotional development probably also needs the correct stimulating at the right time. We can observe evidence of this in those rare cases of children who have been completely neglected from birth. Sometimes illegitimate children in the country are put away in barns to conceal their existence from the neighbors. They are taught nothing, not even to be house-trained or to speak. These children then grow up mentally defective. Even if they are found and rescued by the age of four or five, it is too late. Though they can be taught a lot, their speech is always inadequate and their brains never reach the standards of their brothers or sisters.1.  What can we learn from the first paragraph about the critical period? ______