According to the passage, brain differences are caused by ______ factors.

A．Brain differences indicate superiority of one gender over the other.
B．Results of scientific research can not support popular belief.
C．The difference in the brain structure between the men and women has been known for a long time.
D．Biologists are conducting research where psychologists have given up.

A．activities carried out by the brain
B．the brain structure of men and women
C．school subjects
D．skills of men and women