Why do mitochondria and chloroplasts have relatively small genomes?

a) their bacterial ancestors had small genomes
b) over time, much of their nucleic acid has migrated to the host cell nucleus
c) they do not need many genes to function
d) they lack non-coding dna

B) Over time, much of their nucleic acid has migrated to the host cell nucleus

Explanation:

The origin of chloroplasts and mitochondria is explained via endosymbiosis theory: they  originated as prokarytoic symbionts. The host cell acquired genetic information from them and “took it up” by lateral gene transfer. Those genes (now located in the eukaryotic cell nucleus-nuclear genome) encode the great majority of chloroplast and mitochondrial proteins, as well as many proteins of the nucleus and cytosol.  

Yet ,chloroplasts and mitochondria retained some of the genes in their genomes.