【正确答案】正确答案：Input hypothesis claims that "Human acquire language in only one way—by understanding messages or by receiving comprehensible input". According to Krashen, input that is useful for L2 acquisition must be neither too difficult nor too easy to understand and ought to be tuned just right to learner's current level, represented as i. In the course of acquiring the L2, learners progress from one level to another. The next level is called "i +1". For i +1 to occur, the input has to be slightly beyond the level at which learners are well proficient. The gap between i and i + 1 is bridged by comprehensible input, which is the information drawn from the context and previous experience. Comprehensible input is considered both necessary and sufficient for L2 acquisition to occur and output plays little role. In other words, L2 learning takes place because the data to which learners are exposed contain comprehensible input. According to Krashen's acquisition-learning hypothesis, there are two ways to approach language learning: acquisition and learning. Acquisition helps us produce natural, rapid, and fluent speech. Learning, which is a conscious study of form, helps us edit this speech. In other words, when we learn something it won't help us produce fluent communication, but it will help us monitor our communication and correct minor errors. It can only occur if three conditions are fulfilled: The performer has to have enough time; the performer has to be thinking about correctness and the performer has to know the rule. Learners will be most likely to use the Monitor in formal exam situations, where their attention has been drawn to linguistic form, and where they have enough time. If all these conditions are fulfilled, the Monitor may be used, but may be used inaccurately.