Which of the following contributes most directly to the hardness of bones? 1) hydroxyapatite 2) periosteum 3) articular cartilage 4) osteoid

The correct option is number 1. Hydroxyapatite contributes most directly to the hardness of bones

Explanation:

Hydroxyapatite can be described as a naturally occurring biological molecule of high importance. It provides hardness the bones and teeth. Hence, option 1 is correct.

Periosteum can be defined as the outer layer of a bone which functions as an attachment site for the skeletal muscles. Osteoid is a structure that is made before a bone occurs in its mature form. Hence all the other options 2, 3 and 4 are false.

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.