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正确答案：
input[解析] Krashen uses the input hypothesis to explain the relationship between language input and language acquisition and to answer the question of how people acquire languages.
自然法倡导人克拉申提出语言输入假设，该假设解释了语言输入与语言习得直接的关系，并对于人们如何习得语言做出了解释。