The population of Megacity, a sprawling metropolis inRemsland, has grown at a fairly steady rate for over a century. A hundred years ago, poor sanitation in thecity caused high mortality rates among the city's inhabitants, and what fueledthe population increase was immigration from rural villages. This immigration has continued and evenincreased. Moreover, in recent decades,city sanitation has improved enormously. Yet the city's population growth has not significantly accelerated.
Which of the following, if true, most helps to explainwhy the city's population growth rate has not changed?
(A) Mortality rates that were associated with poorsanitation in Megacity a hundred years ago were orders of magnitude higher thanare mortality rates associated with vehicular traffic, which is currently amajor cause of death in the city.
(B) For several decades, Megacity, as distinct from thecountryside, has had a steadily declining birth rate.
(C) Cities smaller than Megacity have also experiencedsustained population growth.
(D) The great majority of immigrants to Remsland settlein Megacity, at least initially.
参考答案: B
思路：文章中说immigration变多, 说明外来移民到Megacity的人多了,sanitation has improved enormously, 说明死的人变少了” 那为什么人口不增加呢? 目前又两种可能, 移民出去的人变多了,
新增人口(除移民来Megacity的外,就只剩新出生的)变少了.
选项分析：A：这是一个状态比较, 没有变化;
C：小于Megacity的城市与本题无关;
D: 大量的移民在Megacity定定居了, 这应该说明Megacity的人口上升, 与现象不符;
E: 没有变化,又在比较
B: 出生率在下降, 是变化. 此选项告诉我们, 虽然死的人少了,并且移民来的人多了, 但是新出生的少了, 所以人口保持没有增加. 正确答案
知识点：解释不变化，解释矛盾现象