题目内容：
根据下面材料，回答题。Man has been storing up useful knowledge about himself and the universe at the rate which has been spiraling upward for 10，000 years．
The _____21_____ took a sharp upward leap with the invention of writing，but even ____22______ it re-mained painfully slow for several centuries．In Europe the next great leap forward_____23_____knowl-edge acquisition did not occur _____24_____ the invention of movable type in the l5th century by Guten-berg and others．______25____ to 1，500，by the most optimistic_____26_____，Europe was producing books at a rate of 1，000 titles per year．This means that it ____27______ a full century to produce a library of100，000 titles．By l950，four and a half ____28______ later．the rate had accelerated so sharply that Eu．rope was producin9 120，000 titles a year．____29______once took a century now took only ten months．
By 1960，a_____30_____ decade later，the rate had made another significant jump，_____3l_____a century’s work could be finished in seven and a half months． ____32______ ，by the mid．sixties．the output of books on a world____33______，Europe included，a pproached the prodigious figure of 900 titles per day．One can _____34_____ argue that every book is a net gain for the advancement of knowledge．Never-theless we find that the accelerative _____35_____ in book pubfication does．in fact．crudely _____36_____ the rate at which man discovered new knowledge．For example，prior to Gutenber9 ______37____ 11 chemi．cal elements were known．Antimony，the 12th，was discovered _____38_____about the time he was working on his invention．It was fully 200 years since the11 th，arsenic，had been discovered．_____39_____ the same rate of discovery continued，we would by now have added only two or three addi．tional elements to the periodic table since Gutenberg．_____40_____，in the 450 years after his time，certain people discovered some seventy additional elements．And since l900 we have been isolating the remaining elements not at a rate of one every two centuries，but of one every three years．
第21题答案为__________ A．accumulation
B．development
C．knowledge
D．rate
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