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

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

A.People in western Africa did not develop staple crops that they stopped cultivating once rice and yams were introduced.

B.There are no plants native to western Africa that, if domesticated, could serve as staple food crops.

C.Rice and yams were grown as staple crops by the earliest agricultural societies outside of western Africa.

D.Kola and okra are better suited to growing conditions in western Africa than domesticated rice and yams are.

E.Kola and okra were domesticated in western Africa before rice and yams were introduced there.
题目没看懂，根据有关无关做错了。
答案是A，牛牛给解释一下什么意思。
  还有C怎么不对，文中有elsewhere正好贺outside对应，而且也提到了rice和yam