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．the brain structure as a whole
B．the differences between men and women
C．the functioning of parts of the brain
D．the effects of the trunk line