GWD19-40，答案好像反了。

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.