<Tr> <Td> D Ring </Td> <Td> 66,900--74,510 </Td> <Td> 7,500 </Td> <Td> </Td> </Tr> <Tr> <Td> C Ring </Td> <Td> 74,658--92,000 </Td> <Td> 17,500 </Td> <Td> </Td> </Tr> <Tr> <Td> B Ring </Td> <Td> 92,000--117,580 </Td> <Td> 25,500 </Td> <Td> </Td> </Tr> <Tr> <Td> Cassini Division </Td> <Td> 117,580--122,170 </Td> <Td> 4,700 </Td> <Td> Giovanni Cassini </Td> </Tr>

Explain why saturn's f-ring is confined to such a narrow band