Adults are often surprised by how well they remember something they learned as children but have never practised in the meantime. A man who has not had an opportunity to go swimming for years can ___21___ swim as well as ever when he gets back in the water. He can get on a bicycle after several decades and still 22 away. A mother who has not 23 the words for years can teach her daughter the poem that begins 'Twinkle, twinkle, little star' or recite the story of Cinderella or Snow White. One explanation is the 24 of over learning, which can be stated as following: Once we have learned something, additional learning increases the 25 of time we will remember it. In childhood, we usually continue to practise such skills as swimming, bicycle riding long after we have learned them. We continue to listen to and 26 ourselves of poems such as 'Twinkle, twinkle, little star' and childhood tales such as Cinderella or Snow White. We not only learn but __27 . The law of over learning explains why cramming (突击学习) for an examination, though it may result in a(an) 28 grade, is not a 29 way to learn a school course. By cramming, a student may learn the subject well enough to get by on the examination, but he is likely soon to forget almost everything he learned. A little over learning, 30 , is usually a good investment toward the future. 21. A. only               B. hardly           C. still               D. even 22. A. move              B. drive             C. travel             D. ride 23. A. thought about     B. cared for         C. showed up      D. brought up 24. A. result             B. law              C. rule             D. cause 25. A. accuracy          B. unit                 C. limit             D. length 26. A. remind            B. inform             C. warm            D. recall 27. A. recite              B. overlearn         C. research         D. improve 28. A. passing           B. average             C. excellent             D. discouraging 29. A. convenient       B. demanding       C. satisfactory       D. swift 30. A. at most          B. by the way       C. on the other hand  D. in the end