Researchers Found That a Certain Species of Sea Slug, Elysia

Researchers found that a certain species of sea slug, Elysia chlorotica, has in its genome a gene for repairing and maintaining chloroplasts.The gene is identical to one found in the algae from which the slugs steal chloroplasts for performing photosynthesis during periods of dietary shortfall.How did the slugs get the gene?

A) through vertical gene transfer
B) through horizontal gene transfer
C) through diagonal gene transfer