【正确答案】正确答案：Duality makes our language productive. A large number of different units can be formed out of a small number of elements—for instance, tens of thousands of words out of a small set of sounds. And out of the huge number of words, there can be astronomical number of possible sentences and phrases, which in turn can combine to form unlimited number of texts. Most animal communication systems do not have this design feature of human language. If language had no such design feature, then it would be like animal communicational system which consists of only a number of basic sounds and this would be highly limited. Then we would not be able to produce a very large number of sound combinations (e. g. words), which are distinct in meaning. In other words, the number of messages one can send would be restricted to the number of basic sounds.