In US V United States Gypsum, Regarding Sharing of Price Information by Information

In U.S. v. United States Gypsum, regarding sharing of price information by gypsum makers, the Supreme Court held that:

A) a rule of reason applied to the case, which the arrangement failed
B) the case was subject to a per se rule against price fixing, so it was illegal
C) the gypsum industry is exempt from antitrust laws
D) the gypsum industry is so small information sharing is essential
E) the gypsum industry is so small that it simply could not conspire to restrain information