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. The response variable in this experiment is:.
A, the different habitat locations.
B. the crabs
C. the season each sample was taken.
D. the weight of the crab