[单项选择]

The invention of both labor-saving tools and tools of intelligence is rarely accidental. Instead, it is usually the product of human need; (21) is truly the mother of invention. People usually devise tools to (22) for natural deficiencies. For example, people invented weapons to defend (23) from physically superior (24) . But (25) is only one incentive for inventions. People also invent (26) tools to (27) certain established tasks more efficiently. For instance, people developed the bow and arrow from the (28) spear or javelin in order to shoot (29) and strike with greater strength.
(30) civilizations developed, greater work efficiency came to be demanded, and (31) tools became more (32) . A tool would (33) a function until it proved (34) in meeting human needs, at which point an improvement would be made. One impetus for invention has always been the A. but
B. and
C. while
D. thus