Reading skills among high school students in Gotham have bee

[BTGmoderatorAT]

Reading skills among high school students in Gotham have been steadily declining, which can only be the result of overcrowding in the schools.

Which of the following, if true, most seriously weakens the argument expressed above?

a) The high school system in Gotham succeeds in giving students a good education at considerably less cost than do most systems.
b) Several cities have found that overcrowding in the schools is not always associated with lower reading scores.
c) Gotham schools have a greater teacher-to-student ratio than most other school systems.
d) Students' reading skills have not declined in other cities where the high schools are just as crowded as those of Gotham.
e) Schools are not overcrowded in many cities where high school reading scores have declined more than they have in Gotham.

What's the best approach to determine the answer? Can any experts help?