75．What is the main idea of the passage?
(A)Air travel benefits people and industries．
(B)Train Travel has some advantages over air travel．
(C)Great changes have taken place in modern travel.
(D)The high speed of air travel is gained at a cost.

本文出现的关键词airplane，排除BC，文章并不是侧重于乘飞机的优点，而是人们为了追求速度乘飞机所付出的代价。