Why Do Symbionts Often Contain Lower G+C Content When Compared

Why do symbionts often contain lower G+C content when compared to free- living organisms?

A) Replicating high G+C content genomes requires several additional enzymes, to break apart the strong triple bonds formed from GC pairs, which are rarely found in symbionts with relatively small genome sizes.
B) Symbionts usually do not have as many DNA repair mechanisms as free- living organisms, and two common spontaneous mutations change GC pairs into AT pairs.
C) Free- living organisms must have a more stable genome to survive; low G+C content organisms are often selected against in nature.
D) Ancestral symbionts had low GC content genomes by chance, and these low GC content genomes are passed onto their progeny.