What Are Two Proposed Causes for the Cambrian Explosion

What are two proposed causes for the Cambrian Explosion?

A) A meteor killed off the dinosaurs, freeing up space for many other living things, resulting in much higher biodiversity.
B) A sufficiently high level of oxygen in the atmosphere and the development of hard parts for protection from predators.
C) Photosynthesis had used up enough carbon dioxide from the atmosphere that more active animals could survive.
D) The ice age ended and the climate warmed up enough to support the huge increase in biodiversity.
E) The cooling Earth's core finally reached a temperature that allowed the continents to stabilize such that they could move around but still hold together.