Man has been storing up useful knowledge about himself and the universe at the rate which has been spiraling upward for 10,000 y

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问题     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【C1】______took a sharp upward leap with the invention of writing, but even【C2】______it remained painfully slow for several centuries. The next great leap forward【C3】______knowledge acquisition did not occur【C4】______the invention of movable type in the 15th century by Gutenberg and others.【C5】______to 1500, by the most optimistic【C6】______Europe was producing books at a rate of 1000 titles per year. This means that it【C7】______a full century to produce a library of 100,000 titles. By 1950, four and a half【C8】______later, the rate had accelerated so sharply that Europe was producing 120,000 titles a year.【C9】______once took a century now took only ten months. By 1960, a【C10】______decade later, the rate had made another significant jump,【C11】______a century’ s work could be finished in seven and a half months.【C12】______by the mid-sixties, the output of books on a world【C13】______, Europe included, approached the prodigious figure of 900 titles per day.
    One can【C14】______argue that every book is a net gain for the advancement of knowledge. Nevertheless we find that the accelerative【C15】______in book publication does, in fact, crudely【C16】______the rate at which man discovered new knowledge. For example, prior to Gutenberg 【C17】______11 chemical elements were known. Antimony, the 12th, was discovered【C18】______about the time he was working on his invention. It was fully 200 years since the 11th, arsenic, had been discovered.【C19】______the same rate of discovery continued, we would by now have added only two or three additional elements to the periodic table since Gutenberg.【C20】______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.
【C19】

选项 A、As
B、Had
C、If
D、With

答案B

解析 本句是表示与过去事实相反的虚拟语气,省略if,助动词had提前。
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