How did Earth form according to the given material?

• A. A protoplanet collided with young Earth to form new Earth.
• B. The Sun absorbed Earth.
• C. Earth formed from gas identical to the Sun.
• D. Earth formed by asteroid fragmentation.

Answer: (A)

Earth formed through collisions and accretion of material in the young solar system. A protoplanet colliding with the early Earth merged with it, adding mass and helping build the planet we know today. This fits the idea that planets grow by successive impacts and sticking together of smaller bodies in the protoplanetary disk. In fact, a well-known version of this story describes a giant impact where a Mars-sized body struck the proto-Earth, producing debris that eventually formed the Moon while the remaining mass became Earth. The other ideas don’t describe how a rocky planet forms: the Sun absorbing Earth would remove it, Earth wouldn’t form from gas with solar composition, and asteroid fragmentation would break apart material rather than assemble a planet.