Define light-year, and explain how and why light-years are used to measure distances in the universe.

A light-year is how astronomers measure distance in space. It's defined by how far a beam of light travels in one year – a distance of six trillion miles. ... First, light is convenient. Throughout the universe, all light travels at exactly the same speed: about 670 million miles per hour.

Explanation:

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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