A Strength of Nisbett and Cohen's Research Program into the Regional

A strength of Nisbett and Cohen's research program into the regional differences in violence between the U.S.South and North is that

A) they accounted for biases between the two groups.
B) they were able to manipulate all of their independent variables.
C) they were able to cause northerners to respond like southerners.
D) there are no response bias differences between these two groups.
E) they used multiple methods across their studies.