The backbone is made up of many small bones called

The backbone is not a single bone, but a column of drum-shaped bones called vertebrae (singular, vertebra). There are 33 vertebrae altogether, although some of these join together as the body grows. The vertebrae are separated by disks of rubbery material called cartilage. Cartilage is very important because it cushions the bones when you run or jump.  

the earth is thought to have been formed about 4.6 billion years ago by collisions from a cloud of hydrogen, helium, rock, ice and other materials. these were pulled together by gravity to form objects in and around earth like- asteroids and small early planets called planetesimals.

so, the best possible answer to this question will be option d- to harden materials to form different parts of earth’s layers