The earliest models of the solar system employed what Aristotle, and Plato before him, had taught was the perfect form: the circle. The simplest possible arrangement-uniform motion around a circle having Earth as its center-provided a fairly good approximation to the orbits of the Sun and the Moon. But it could not account for the observed variations in planetary brightness or their retrograde motion. Thus, a more complex model was needed to describe the motion of the planets. Eric Chaison and Steve McMillan, Astronomy Today, 3^rd ed. A) Argument; conclusion: A more complex model ... motion of the planets. B) Argument; conclusion: But it could not account ... their retrograde motion. C) Nonargument. D) Argument; conclusion: The simplest possible arrangement ... Sun and Moon. E) Argument; conclusion: The earliest models of the solar system ... the circle.

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The passage presents an argument that begins with the premise that early models of the solar system were based on circular orbits centered on Earth, and concludes that a more complex model was needed to accurately describe planetary motion, due to the inability of the simple model to account for observed phenomena like variations in planetary brightness and retrograde motion.

The Earliest Models of the Solar System Employed What Aristotle