What was the main result of the joining of planetesimals in Earth's evolution?

Correct answer: Gradual increase in Earth's size

The direct answer is B: joining planetesimals gradually increased Earth’s size and mass. Planetesimals were small solid bodies formed from material in the early solar system. Their repeated collisions and gravitational attraction caused accretion, meaning that smaller bodies stuck together and built a larger planetary body. The reasoning is: dust and rock fragments collect → planetesimals form → planetesimals collide and join → mass increases → the growing Earth becomes larger. Option B is correct because it describes the immediate broad result of accretion. Option A is wrong because oceans did not form immediately; Earth first became extremely hot, and oceans appeared later after cooling and condensation. Option C is wrong because modern humans evolved much later and are unrelated to the joining of planetesimals. Option D is wrong because the ozone layer did not form immediately; significant ozone depends on atmospheric oxygen and later photochemical processes. The word “gradual” is important: planetary growth occurred through many additions over time, not in one instant. Memory cue: accretion means “adding pieces to make a bigger planet.”