During the Collapse of a Massive Star the Core Will

During the collapse of a massive star the core will rebound outward in a process called "core bounce". What causes this?

A) A flood of neutrinos is released and pushes outward on the core.
B) A flood of high-energy gamma rays is released and pushes outward on the core.
C) The collapsing core reaches the density of nuclear matter and stiffens.
D) Radioactivity develops because of the tremendous amount of energy available, and the radiation reverses the movement of the core.