What Is the PRIMARY Reason That Psychologists Study Aspects of the Damaged

What is the PRIMARY reason that psychologists study aspects of the damaged brain?

A) Psychologists already know how the normal brain works.
B) Psychologists can learn about normal brain processes by studying what happens when these processes are disrupted.
C) Psychologists usually only have access to persons with brain dysfunction.
D) Psychologists are only interested in abnormal behaviour and the impaired brain processes that produce this behaviour.