<Tr> <Th> 2000 </Th> <Td> 348,189 </Td> <Td>--</Td> </Tr> <Tr> <Th> </Th> <Td> 350,705 </Td> <Td> + 0.7% </Td> </Tr> <Tr> <Th> 2006 </Th> <Td> 353,837 </Td> <Td> + 0.9% </Td> </Tr> <Tr> <Th> 2008 </Th> <Td> 356,730 </Td> <Td> + 0.8% </Td> </Tr>

Which of the following geographical factors helped make the midwest a center for trade