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What does the orbital radius refer to?

  1. The smallest unit of an element.

  2. A cloud of gas and dust in space.

  3. The average distance between the sun and an orbiting object.

  4. An element in Group 1 of the periodic table.

The correct answer is: The average distance between the sun and an orbiting object.

Orbital radius refers to the average distance between the sun and an orbiting object. Options A and D are incorrect because they refer to elements, not distances. Option B is incorrect because it refers to a cloud of gas and dust, not a specific distance. Additionally, not all orbiting objects are surrounded by a cloud of gas and dust. Thus, the correct answer is C.