GWD-9-Q40:

Agricultural societies cannot exist without staple crops.Several food plants, such as kola and okra, are known to have been domesticatedin western Africa, but they are allsupplemental, not staple, foods. All the recorded staple crops grown in westernAfrica

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

A.   People in western Africadid not develop staple crops that they stopped cultivating once rice and yamswere introduced.

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

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

D.   Kola and okra are better suited to growingconditions in western Africa than domesticatedrice and yams are.

E.    Kola and okra were domesticated in western Africa before rice and yams were introduced there.

我选的B，答案是A
B应该错在题目中已经强调了所有的主食，选项中的意思与题中的意思重复
A应该怎么解释呢？好像之前的帖子关于这道题的很少
求各路NN解释！