The difficulty with the proposed high-speed train line is that a used plane can be bought for one-third the price of the train line, and the plane, which is just as fast, can fly anywhere. The train would be a fixed linear system, and we live in a world that is spreading out in all directions and in which consumers choose the free-wheel systems (cars, buses, aircraft), which do not have fixed routes. Thus a sufficient market for the train will not exist.
Which of the following, if true, most severely weakens the argument presented above?

(A) Cars, buses, and planes require the efforts of drivers and pilots to guide them, whereas the train will be guided mechanically.
(B) Cars and buses are not nearly as fast as the high-speed train will be.
(C) Planes are not a free-wheel system because they can fly only between airports, which are less convenient for consumers than the high-speed train's stations would be.
(D) The high-speed train line cannot use currently underutilized train stations in large cities.
(E) For long trips, most people prefer to fly rather than to take ground-level transportation.

I agree with the OA.
However, it is tricky to find the conclusion (in blue) because the sentence in red seems to be the conclusion if you don't pay enough attention (It is at the end of the argument and used the word "thus").
In this sense, I would like to know whether there is a method or approach to find the conclusion in this type of tricky questions in a faster way. CR Powerscore Bible suggests to organize the ideas to find what statement is consequence of another statement; however, it takes time. Thanks!

OA is C.