Select the correct statement about current hypotheses for the cause of the Cambrian explosion. A. Oxygen levels reached a critical threshold at the start of the Cambrian explosion, making the evolution of big, mobile animals possible.
B. With the evolution of algae, oxygen was produced by oxygenic photosynthesis for the first time in the history of life on Earth.
C. Key developmental toolkit genes arose early in the Cambrian, leading to the Cambrian explosion of animal phyla.
D. There are no plausible hypotheses accounting for the rapid adaptive radiation known as the Cambrian explosion.