从以下词语中选择填空, 填写小写: design feature convention arbitrariness duality 1. _________ is one of the design features of human language which refers to the phenomenon that language consists of two levels: a lower level of meaningless individual sounds and a higher level of meaningful units. 2. __________ refers to the fact that the forms of linguistic signs bear no natural relationship to their meanings. 3. ___________ refers to a regularity in the behavior of members of a given group who reeatedly find themselves confronted by a problem of co-ordination, who solve this problem in one of several possible ways, and in return epect the response by others in the group 4. __________ are the features that define our human languages.