18. It is generally believed that the chief reason for the increase in population in developed countries is not so much the rise in birth rates as the decline in death rates as a result of the improvement in medical care.

　　普遍認為，在發達國家人口增長的主要原因與其說是出生率的上升，還不如說是由于醫療保健的改善使死亡率下降了。

　　19. There is no doubt that the increase in demand caused the rise in prices.

　　毫無疑問，需求的增長導致了價格的上漲。

　　20. Additional social stresses may also occur because of the population explosion or problems arising from mass migration movements-themselves made relatively easy nowadays by modern means of transport.

　　由于人口的猛增或大量人口流動(現代交通工具使這種流動相對容易)引發的種種問題也會對社會造成新的壓力。(超循環背誦大表)