请同学帮忙解答一下prep里的一道逻辑题

夜恋宸00

以下是题目以下是题目
The population of Megacity, a sprawling metropolis in Remsland, has grown at a fairly steady rate for over a century. A hundred years ago, poor sanitation in the city caused high mortality rates among the city's inhabitants, and what fueled the population increase was immigration from rural villages. This immigration has continued and even increased. Moreover, in recent decades, city sanitation has improved enormously. Yet the city's population growth has not significantly accelerated.
how to explain
(A) Mortality rates that were associated with poor sanitation in Megacity a hundred years ago were orders of magnitude higher than are mortality rates associated with vehicular traffic, which is currently a major cause of death in the city.
(B) For several decades, Megacity, as distinct from the countryside, has had a steadily declining birth rate.
(C) Cities smaller than Megacity have also experienced sustained population growth.
(D) The great majority of immigrants to Remsland settle in Megacity, at least initially.
(E) Megacity has long offered better employment prospects than most rural areas.
我选的是a理由是人口增长等于出生减去死亡，a答案我的理解是现在存在另一个致使死亡增加的因素，因此即使相比起一百年前人口移民增加，卫生状况变好，但人口增长率依旧不会变化。但是答案是b，有同学能帮我解答一下吗，谢谢

弥小岛

我的理解是 人口增长率=本地人口出生率+外来人口迁移率-死亡率  原文的意思很明显把另外两个变量控制了，sanitation对应死亡率降低，外来人口迁移率依旧上升，那就只剩出生率了。 A明显和原文conflict  和sanitation提升对冲  在explain里属于无关选项     讲的不好 NN来讲吧

RyanR

(A) Mortality rates that were associated with poor sanitation in Megacity a hundred years ago were orders of magnitude higher than are mortality rates associated with vehicular traffic, which is currently a major cause of death in the city.
这句话的意思是过去因为卫生条件差所造成的死亡率比现在因为交通事故造成的死亡率还牛逼很多倍。
那造成死亡率这么高的牛逼东西都解决了，人口增长还不快速上升，不是更不能说明为什么会发生现在的状况了。

lludwig

我的理解是，虽然交通事故率确实很高，但它没有增长，注意文章中说的是growth rate 并没有显著的加速，这种变化是一种静态的变化
但B中所说的是一个动态的变化，即出生率一直再降低，这是可以作为减慢population growth rate的原因de

梦季

看一下！