Approximately 360 Million Years Ago, Flying Insects Reached Massive Sizes

Approximately 360 million years ago, flying insects reached massive sizes (by today's standards) . Why are modern-day insects so small compared to their ancestors?

A) Larger insects were more susceptible to predation; as more predators evolved on Earth, insects became smaller in size.
B) The first flowering plants that appeared on Earth were small in size. Insects evolved to utilize these food sources, becoming smaller in the process.
C) Three hundred sixty million years ago, Earth's atmosphere had much more oxygen. Today's lower oxygen levels cannot support large insects; therefore smaller insects were selected for.
D) As tetrapods appeared on Earth, these animals competed with insects for food sources. As the amount of food decreased, so did the size of most insects.
E) Flying insects actually never attained large sizes; large sizes are not conducive to flight, especially in animals that possess exoskeletons.