According to Stephen Krashen，the formula i + 1 means__________.

According to Stephen Krashen，the formula i + 1 means__________.

A.input contains structures slightly below the learner's present level

B.output contains structures slightly above the learner's present level

C.input contains structures slightly above the learner's present level

D.output contains structures slightly below the learner^ present level

正确答案是C