Why Have Gene Families Evolved? Choose the Most Accurate Answer

Why have gene families evolved? Choose the most accurate answer for gene families in general.

A) Although in some cases researchers have inferred that gene families are adaptive, gene families may also evolve due to genetic drift.
B) Having multiple copies of a gene can increase transcription and translation of the gene, so when increased transcription is adaptive, then natural selection favors genes duplicating into gene families.
C) Gene families are adaptive because each gene can produce a slightly different protein, which allows for diverse functions that are favored by natural selection.
D) Gene families evolve because of the activity of mobile genetic elements, which cause repeated gene duplication events.