Which of the following statements about planetary rings is not true?

a. They can form due to tidal disruption of a moon.  

b. They can form due to dust and chips broken off of a moon.

c. They are usually very thin, as little as 10 km thick.

d. They are seen today just as they were billions of years ago.

e. They can be kept in line by gravitational tugs by shepherd moons.