Astronomers recently found another ring around the planet Saturn   -- and it's the biggest one yet. This newly discovered ring stretches about 15 million miles across, which makes a loop big enough to fit a billion Earths inside. It may be even bigger and reach 22 million miles across at its diameter  . Saturn's new ring is now the biggest known ring in the solar system.  This ring, like Saturn's other rings, is not one solid piece of rock. Instead, it is made up of millions or billions of tiny rocks that orbit   the planet all together and give the appearance of a unified   ring. Unlike the more visible rings, however, this big one is mysterious -- the pieces are far apart, and the ring cannot be seen from the surface of the Earth. If all the pieces were stuck together, they'd be slightly bigger than a rock a half-mile wide. Astronomers suspect that the newly discovered ring gets its material from Saturn's outermost   moon, Phoebe. For example, the dust in the ring circles Saturn in the same path and the same direction as Phoebe. Plus, the surface of Phoebe is marked with holes and places where rock has been knocked away, the result of collisions between Phoebe and other moons and rocks. These collisions could have sent dust out into space to become part of the big ring. The discovery of the ring could help explain other mysteries surrounding Saturn and its dozens of moons. The moon called Iapetus, for example, is two-toned: one side is gray; the other more reddish. Iapetus is within the newly discovered ring, but it orbits in the opposite direction. As a result, some researchers say, debris   from the ring may have coated and color1ed one side of Iapetus as it moved through space, and left the other side alone. "The cause of Iapetus' strange color1 distribution ... has been a puzzle that now seems solved," said Joseph Burns, a scientist. But not everyone agrees. Some think the ring alone may not be able to explain Iapetus' funny color1ing. The main reason why one side of Iapetus is gray may be that   _  .
Saturn's moons move in the same direction there are many moons circling around Saturn debris from the biggest ring covers one side of Iapetus Iapetus strikes against other moons while moving
Question: Astronomers recently found another ring around the planet Saturn   -- and it's the biggest one yet. This newly discovered ring stretches about 15 million miles across, which makes a loop big enough to fit a billion Earths inside. It may be even bigger and reach 22 million miles across at its diameter  . Saturn's new ring is now the biggest known ring in the solar system.  This ring, like Saturn's other rings, is not one solid piece of rock. Instead, it is made up of millions or billions of tiny rocks that orbit   the planet all together and give the appearance of a unified   ring. Unlike the more visible rings, however, this big one is mysterious -- the pieces are far apart, and the ring cannot be seen from the surface of the Earth. If all the pieces were stuck together, they'd be slightly bigger than a rock a half-mile wide. Astronomers suspect that the newly discovered ring gets its material from Saturn's outermost   moon, Phoebe. For example, the dust in the ring circles Saturn in the same path and the same direction as Phoebe. Plus, the surface of Phoebe is marked with holes and places where rock has been knocked away, the result of collisions between Phoebe and other moons and rocks. These collisions could have sent dust out into space to become part of the big ring. The discovery of the ring could help explain other mysteries surrounding Saturn and its dozens of moons. The moon called Iapetus, for example, is two-toned: one side is gray; the other more reddish. Iapetus is within the newly discovered ring, but it orbits in the opposite direction. As a result, some researchers say, debris   from the ring may have coated and color1ed one side of Iapetus as it moved through space, and left the other side alone. "The cause of Iapetus' strange color1 distribution ... has been a puzzle that now seems solved," said Joseph Burns, a scientist. But not everyone agrees. Some think the ring alone may not be able to explain Iapetus' funny color1ing. The main reason why one side of Iapetus is gray may be that   _  .
A. Saturn's moons move in the same direction
B. there are many moons circling around Saturn
C. debris from the biggest ring covers one side of Iapetus
D. Iapetus strikes against other moons while moving
Answer: