When environmental conditions are unfavorable, why is it an advantage for green algae to reproduce sexually?

AThey are multicellular when conditions are unfavorable, which favors sexual reproduction.

BSexual reproduction produces larger numbers of offspring in less time.

CSexual reproduction produces offspring that have greater genetic variety and durable zygotes that can help green algae species survive unfavorable conditions.

DSexual reproduction produces multicellular algae instead of unicellular algae. Multicellular algae are better able to survive harsh conditions.