What term or terms do historans use to refer to a general period of time

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

Physical features of central america look at the map above which of the following is true

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