​What Suggests That People with a Smaller Than Average Hippocampus

​What suggests that people with a smaller than average hippocampus have increased risk of PTSD?

A) Comparisons of the brains of hundreds of people with and without PTSD
B) ​Studies of soldiers who had brain scans prior to enduring a traumatic experience
C) ​Comparisons of the brains of monozygotic twins with and without PTSD
D) ​Studies of the brains of people before and after treatment for PTSD