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 statement 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 schools is not always associated with lower reading scores.
C) Gotham schools have a greater teacher-to-student ratio tham 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.

My question is if Y is result of X, does it mean X causes Y?