The answer is C, because there are only so many elements therefor you can have only so many compounds.
The correct option is SKELETAL SYSTEM.
Calcium is a mineral that is found in many food and the body needs it to maintain strong bones and teeth and to carry out some biochemical reactions in the body. Calcium in the body is stored in bones and teeth; calcium supports the structure and hardness of these body parts.
<span>d. our atmosphere is constantly being heated by the sun
Radiation is heat transfer from a source through space. </span>