1. according to the nebular theory, the solar system formed from ?

a. accretion of protoplanets.
b. a large, thin cloud of dust and gas.
c. colliding planetesimals.
d. all of the above

2. which of the following statements explains why the terrestrial planets are small and rocky instead of large and less dense like the gas giants?

a. the inner planets were exposed to much higher temperatures.
b. ice-forming compounds vaporize at high temperatures.
c. rock-forming materials can condense at high temperatures.
d. all of the above

1. The correct answer is B. The nebular theory states that the solar system is the result of the collapse, for unknown reasons, of a large, thin cloud of dust and gas. After this collapse, the pockets of gas and dust started gathering into denser regions. As these regions collected more and more matter, they started to rotate, which led to the accumulation of a big ball in the center, the Sun, and flattenedmatter around, the protoplanetary disc. Then the planets got formed by the accretion of this disc, which eventually created the planets. 

2. The correct answer is D. The terrestrial planets were formed by accretion of material that was denser in its constitution. This is the reason why they remained closer to the sun - they were heavier than gas, that is the material the Jovian planets are mostly made of.