题目内容：
Read the following text. Choose the best word(s) for each numbered blank and mark A , B , C or D on ANSWER SHEET1.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 remained painfully slow for several centuries. The next great leap forward _________ 23 knowledge acquisition did not occur _________ 24 the invention of movable type in the 15th century by Gutenberg and others. _________ 25 to 1500, by the most optimistic _________26 Europe was producing books at a rate of 1000 titles per year. This means that it _________ 27 a full century to produce a library of 100,000 titles.By 1950, four and a half _________ 28 later, the rate had accelerated so sharply that Europe was producing 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, _________31 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, approached the prodigious figure of 900 rifles per day.
One can _________34 argue that every book is a net gain for the advancement of knowledge. Nevertheless we find that the accelerative_________35 in book publication does, in fact, crudely_________36 the rate at which man discovered new knowledge. For example, prior to Gutenberg _________37 11 chemical elements were known. Antimony, the 12th, was discovered_________ 38 about the time he was working on his invention. It was fully 200 years since the llth, arsenic, had been discovered. _________ 39 the same rate of discovery continued, we would by now have added only two or three additional elements to the periodic table since Gutenberg. _________40 , in the 450 years after his time, certain people discovered some seventy additional elements. And since 1900 we have been isolating the remaining elements not at a rate of one every two centuries, but of one every three years. A．accumulation
B．development
C．knowledge
D．rate
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