Proetien. Protein. Protein. Portein. Protein is the correct answer. This is not a very hard question. You could probally find it online if you really tried.
Muscles move body parts by contracting and then relaxing. Muscles can pull bones, but they can't push them back to the original position. So they work in pairs of flexors and extensors. The flexor contracts to bend a limb at a joint.
