千年之交的钟声已经奏响,21世纪的巨轮承载着人们对新世纪的美好憧憬起航。回顾20世纪,科学技术的进步使人类的生活水平发生了天翻地覆的变化。尤其是医学的进步,使婴儿的死亡率下降、人的期 The bell at the turn of the millennium has been ringing. The 21st-century giant wheel carries people’s beautiful vision for the new century. Recalling the twentieth century, the progress of science and technology has brought about a drastic change in the standard of living of humankind. In particular, medical advances have led to declines in infant mortality rates and human periods