An Important Part of Writing a Formula Unit for an Ionic

An important part of writing a formula unit for an ionic compound is to make sure that

A) the charges of the ions add up to zero.
B) there is never more than one type of cation and one type of anion in the formula.
C) the ionic compound actually exists in nature.
D) the anion is written first, followed by the cation.
E) the ratio of cation to anion is 1:1.