资料题：How many significant demographic changes ever occurred in the past million years（　）。

The growth of population during the past few centuries, is no proof that population will continue to grow straight upward toward infinity and doom.On the contrary, demographic(人口统计) history offers .evidence that population growth has not been a all constant.According to paleoecologist(古生态学家)Edward Deevey, the past million years show three momentous change.The first, a rapid increase in population around one million B.C.,followed the innovationsof tool-making and tool-using.But when the new power from the use of tools has been exploited,the rate of: world population growth fell and became almost stable.The next rapid jump in population started perhaps 10,000 years ago, when mankind began to keep bends, plow and plant the earth.Once again when initial productivity gains had been absorbed, the rate of population growth abated.These two episodes suggest the third great change, the present rapid growth, which began in the West between 250 and 350 years ago, may also slow down when,or if, technology begins to yield fewer innovations.0f course, the current knowledge revolution may continue without foreseeable end.Either way,contrary to popular belief in constant geometric growth-population can be expected in the long run to adjust to productivity.And when one takes this view, population growth is seen to represent economic progress and human triumph rather than social failure.