While researching a topic, a student has taken the following notes: Astronomers estimate that the number

Choice D is the best answer. The sentence makes a generalization—that a comet’s orbit around the Sun may change over time—and supports the generalization with the example of the orbit of comet 81P/Wild, which once lay between the orbits of Uranus and Jupiter but is now positioned between the orbits of Jupiter and Mars.

Choice A is incorrect. The sentence emphasizes the number of comets orbiting the Sun and makes a generalization about their orbits, but it doesn’t support the generalization with an example. Choice B is incorrect. The sentence makes a generalization about comets and compares them to the planets Uranus, Jupiter, and Mars; it doesn’t make and support a generalization about comets’ orbits. Choice C is incorrect. While the sentence provides an example of a comet whose orbit has changed, it doesn’t make a generalization about the orbits of comets.