Which best describes the pericardium? structures inside the heart that ensure that blood flows in one direction only a sac around the heart that allows the heart muscle to receive oxygen a sac around the heart that protects the heart and chest tissues structures inside the heart that bring blood to the lungs for oxygenation

The right option is; a sac around the heart that protects the heart and chest tissues.

The pericardium is a double-walled sac that surrounds the heart. The pericardium possesses two layers (a serous layer and a fibrous layer) and it covers the pericardial cavity that contains pericardial fluid. The pericardium protects the heart and chest tissues from infections. It also lubricates the heart, and prevent over dilation of the heart.

 C

Explanation:

the reason the two sea urchins don't compete with each other for food, resources, etc. is because the urchins have adapted to the environment but they have also adapted to each other. this is most likely because the genes of the sea urchins.