A Researcher Was Studying How the Weights of Horseshoe Crabs

A researcher was studying how the weights of horseshoe crabs along the East Coast vary between northern and southern habitats. In spring 2006, during breeding season, the researcher randomly selected 50 horseshoe crabs in southern habitats. In fall 2006, before the breeding season began, the researcher randomly selected 50 horseshoe crabs from northern habitats. It was found that crabs in southern habitats weighed more, so the researcher claimed that location had an effect on the weight of horseshoe crabs. Why should the results of this study not be trusted?

A) The study was not double-blind.
B) The investigator was biased. She knew beforehand what the study would show.
C) The investigator should have used more than two locations.
D) The time of year during which the samples were taken is a confounding variable.