What are the two ways in which central cities can thrive?

The difference(s) between the grand canyon and valles marineris is/are: (a) the grand canyon was formed by water, but valles marineris was not. (a) valles marineris is much longer than the grand canyon. (c) valles marineris is much wider than the grand canyon. (d) valles marineris has large volcanoes near it. all of the above

In africa what do they call a crop grown for direct sale and not for use?

Although romania possesses broad plains with fertile soils, its people a) work in citiesb) are impoverishedc) have emigrated to other nationsd) are entrepreneurs