The correct answer is - A) arthropods evolved before vertebrates did.
The arthropods have evolved much earlier than the vertebrates. They were on the scene millions of years before the earliest marine forms of the vertebrates. Because the arthropods were on the scene much earlier, they managed to use the newly formed suitable environment on the land. They were initially marine, but with only slight adaptations they were able to adapt to the terrestrial life.
Once the arthropods got out of the marine environment and started to take up the niches in the terrestrial environment, they experienced a real explosion in their evolution and diversification. They were the dominant land dwellers, and because of the lack of competition, as well as the higher levels of oxygen, they managed to reach pretty big sizes for their standards. Unfortunately for them, their brain capacity is limited to a very low level, so once the competition came on the scene, they were quickly pushed from the throne as the dominant terrestrial creatures.
