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 titles 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 11th, 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.

【21】

A．accumulation

B．development

C．knowledge

D．rate