The invention of both labor-saving tools and tools of intelce is rarely accidental. Instead, it is usually the product of human need【C1】______is truly the mother of invention. People usually devise tools to【C2】______for natural deficiencies. For example, people invented weapons to defend【C3】______from physically superior【C4】______But【C5】______is only one incentive(鼓励)for inventions. People also invent【C6】______tools to【C7】______certain established tasks more effectively. For instance, people developed the bow and arrow from the【C8】______spear or javelin(标枪)in order to shoot【C9】______and strike with greater strength.【C10】______civilizations developed, greater work efficiency came to be demanded, and【C11】______tools became more【C12】______A tool would【C13】______a function until it proved【C14】______in meeting human needs, at which point an improvement would be made. One impetus(动力)for invention has always been the【C15】______for speed and high-quality results — provided they are achieved【C16】______reasonable costs. Stone pebbles were sufficient to account for small quantities of possessions,【C17】______they were not efficient enough for performing sophisticated mathematics. However, beads(念珠)arranged systematically evolved into the abacus(算盘). The【C18】______of this tool can be【C19】______to the development of commerce in the East around 3,000 BC and the abacus is known【C20】______by the ancient Babylonian, Egyptian, Chinese, etc. 【C1】