What Was the Purpose of "Grandfather Clauses

What was the purpose of "grandfather clauses"?

A) They ensured that all children,white or black,had proper caregivers.When parents weren't available,these clauses granted grandparents legal custody of children.
B) They prevented any black household from having more than two generations living in it at any given time,thereby breaking up black families.
C) They extended the right to vote to those whose relatives were enfranchised before the end of the Civil War,namely whites.
D) They required black public workers to refer to white public workers as "grandfather."