Medical reseachers, who have identified a genetic abnormality in parents of children with down's syndrome, which they believe, instead of the age of the mother, may be the cause of this congenital defect.

a..
b. which, they believe, may be more important than the age of the mother as
c. believe this abnormality and not the age of the mother as being
e. believe that this abnormality, other than how old the mother is, may be
e. believe that this abnormality, rather than the age of the mother, may be

OA E Can you please explain the options and the key take away?