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.
普遍认为，在发达国家人口增长的主要原因与其说是出生率的上升，还不如说是由于医疗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.
毫无疑问，需求的增长导致了价格的上涨。
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动(现代交通工具使这种流动相对容易)引发的种种问题也会对社会造成新的压力。(超循环背诵大表)
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