gwd-9-40 困扰了很久

GWD-9-40

Agricultural
societies cannot exist without staple crops. Several food plants, such as kola
and okra, are known to have been domesticated in western Africa,
but they are all supplemental, not staple, foods. All the recorded staple crops
grown in western Africa were introduced from
elsewhere, beginning, at some unknown date, with rice and yams. Therefore,
discovering when rice and yams were introduced into western Africa
would establish the earliest date at which agricultural societies could have
arisen there.

Which
of the following is an assumption on which the argument depends?

1. People in western Africa did not develop staple crops that they
        stopped cultivating once rice and yams were introduced.
2. There are no plants native to
        western Africa that, if domesticated,
        could serve as staple food crops.
3. Rice and yams were grown as
        staple crops by the earliest agricultural societies outside of western Africa.
4. Kola and okra are better
        suited to growing conditions in western Africa
        than domesticated rice and yams are.
5. Kola and okra were
        domesticated in western Africa before
        rice and yams were introduced there.