According to the research presented in the Critical Thinking box on smoking and psychological disorders, nicotine is implicated in psychological disorders because:

A. nicotine affects many of the same neurotransmitters and brain areas that are involved in mental disorders.
B. people with schizophrenia or depression tend to have abnormally high threshold levels for responding to nicotine.
C. nicotine is a toxic chemical that suppresses the release of dopamine and other neurotransmitters that are necessary for normal mental functioning.
D. people with psychological disorders tend to absorb more nicotine from cigarettes than other people, making it easier for them to become addicted to nicotine.