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, 3rd ed.)
a Argument; conclusion: A more complex model . . . motion of the planets.
b Argument; conclusion: The simplest possible arrangement . . . retrograde motion.
c Nonargument.
d Argument; conclusion: The simplest possible arrangement . . . Sun and Moon.
e Argument; conclusion: The earliest models . . . the circle.