【正确答案】2. Language is not an isolated phenomenon, it is a social phenomenon. Its use, change and development are restricted by various social factors. The relationship between language and society is what sociolinguistics is mainly about. Language is also a mental activity of humankind. The research on such questions as "How do human beings acquire their mother tongue?" and "How is linguistic information is processed and memorized in the human brain?" constitute a branch of linguistics--psycholinguistics. The study of linguistics and its development is closely related to language teaching, especially foreign language teaching. The application of linguistic theories and research results in language teaching is what applied linguistics is all about.