Which statement describes the distinction between inner and outer planets?

• A. Inner planets are rocky and smaller; outer planets are gaseous and larger
• B. Inner planets are gaseous; outer are rocky
• C. All planets are similar in composition
• D. Outer planets are closer to the Sun than inner planets

Answer: (A)

The important idea is that planets fall into two groups by what they’re made of and how big they are, depending on how far they are from the Sun. The inner planets—Mercury, Venus, Earth, and Mars—are rocky and smaller; they have solid surfaces and higher densities. The outer planets—Jupiter, Saturn, Uranus, and Neptune—are much larger and dominated by gases and ices, with thick atmospheres and no solid surfaces. This contrast in composition and size is what the statement captures.

That’s why the other ideas don’t fit: not all planets have similar compositions, so saying they’re all alike is incorrect; outer planets aren’t closer to the Sun, they’re farther away; and inner planets are not gaseous, they’re rocky.