When One Is Defining a Concept

When one is defining a concept:

A) there will only be one possible genus, which is why it is determined first
B) it doesn't matter what genus is chosen, as long as the rest of the definition is consistent with it
C) there might be many choices of genus from which to choose
D) the genus should always include the word "where" or the word "when"
E) the genus should be chosen last