The nebular model explains
a. why the inner and outer planets differ.
b. why the planets are all widely separated.
c. the presence of comets and asteroids.
d. all of the above.

Answer;

All the above;

The nebular model explains all the following;

why the inner and outer planets differ.  why the planets are all widely separated.  the presence of comets and asteroids. Explanation;The nebular theory is a theory that explains the formation of solar systems. According to the theory stars were born from clouds of interstellar gas and dust. The solar nebula is the gaseous cloud from which the Sun and planets formed by condensation. The sun, the planets, and all the objects in the solar system originated or formed from the solar nebula billions of year ago.The theory further explains why the inner and outer planets differ, why the planets are all widely separated and also the presence of comets and asteroids.