question in 400 must-have words for Toefl

dori0214

在这本书的第15页有一篇阅读，总觉的书中给的答案有些蹊跷，有人能告诉我为什么我错了么？

The migration from Asia to North America across the Bering Strait
(perhaps by land bridge) was a monumental event in human history.The
process of overspreading the Americas took more than 1,000 years, or 30
generations.This might seem to confirm common sense—that slow travel
was inherent in any great migration without wheeled
vehicles across unknown terrain. Further thought shows
that this process was remarkably fast—about 10 northsouth
miles per year, on average. The Americas were
populated at an astounding pace, when one considers
the physical limits of the human body and the physical
features of the American continents. Legs of humans can move only so
fast under the best of circumstances, and they work even slower over
mountain passes or deserts. Populations spread through the diverse
regions of the Americas (grasslands, eastern forests, coastal swamps) and
needed to adapt to their new environments.The migrants’ lifestyle had
evolved over the years to that of professional nomads ensuring that they
would find the resources needed to survive.

1. Which sentence best expresses the essential information in this
passage?
a. Human migration across the Bering Strait was remarkably slow.
b. Physical limitations made migration across the Bering Strait almost
impossible.
c. Humans readily adapted to life in the Bering Strait.
d. The migration through the Americas was surprisingly fast.

答案是a，我认为是d