Robert decided to join the army because he failed out of college and he was tired of dealing with his father's mental disorder. After hearing he enlisted, his fiancé broke off their engagement. Boot camp was much more difficult than Robert had anticipated. Robert began to see and hear things that were not present and was diagnosed with schizophrenia. The best explanation for why Robert developed schizophrenia is the __________ theory.

A. social factors
B. recessive genetics
C. multiple stressors
D. vulnerability

Answer: D