Which of the following best describes the composition of a comet?

• A. Pure carbon dioxide crystals
• B. Solid iron and nickel core
• C. Primarily hydrogen and helium gas
• D. A mixture of ice, dust, and rocky material

Answer: (D)

Verified step by step guidance

1. Understand that a comet is a small celestial body composed of various materials, often described as a 'dirty snowball.'
2. Recall that comets are primarily made up of ice (such as water ice, carbon dioxide ice, and other frozen gases), dust, and rocky particles.
3. Recognize that pure carbon dioxide crystals alone do not represent the full composition of a comet, as comets contain a mixture of different ices and solid materials.
4. Note that a solid iron and nickel core is characteristic of some asteroids or planetary cores, not comets.
5. Understand that comets are not primarily composed of hydrogen and helium gas, which are more typical of stars and gas giant planets.