Answer:
(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.
Explanation:
The letter C presents an argument strong enough to weaken more seriously the argument presented above. This is because airplanes offer more convenience, practicality and agility to the group of consumers that are at stake, which are those consumers who prefer freewheeling systems, which do not have fixed routes. In addition to airplanes that fit this pattern, they offer greater convenience and comfort to consumers.