Which of the Following Approaches Would Provide the Strongest Test

Which of the following approaches would provide the strongest test of the hypothesis that the density of individuals affects aggression?

A) A controlled experiment that varies aggression by manipulating hormone level
B) A controlled experiment that varies density and records aggression
C) A correlation between aggression and density across many populations
D) Observations of aggression over a season as density varies
E) A demonstration that aggression is highest just before the breeding season