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. Ttherefore, 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? ['People in western Africa did not develop staple crops that they stopped cultivating once rice and yams were introduced.', 'Kola and okra were domesticated in western Africa before rice and yams were introduced there.', 'There are no plants native to western Africa that, if domesticated, could serve as staple food crops.', 'Kola and okra are better suited to growing conditions in western Africa than domesticated rice and yams are.']