【正确答案】Input hypothesis, advanced by Krashen, a famous American applied linguist, in the late 1970s and the early 1980s is an all-round theory concerning second language acquisition. Meanwhile, it is the most influential and controversial theory in second language acquisition research. Such influences still exist till now. Krashen states that language input plays an identical part with LAD. Together with language input, a learner can construct a series of internal expressions of a good language; in other words, construct an independent language system outside the learner’ s first language. The Input Hypothesis answers the question of how a language acquirer develops competency over time. It states that a language acquirer who at “level I” must receive comprehensible input that is at “level i+1” . We acquire, in other words, only when we understand language that contains structure that is a little beyond where we are now.