请教一道GWD 假设题

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. 结论
            

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.

我选了C，答案是A，请高人指点思路。谢谢

原文大意——

因为——没有日常谷物，就没有农业社会

        西非有记录记载的日常谷物开始于引进大米和芋头。

所以——研究西非何时引进大米和芋头，就能确定西非何时有了农业社会。

很显然，如果西非在引进大米和芋头之前，就有了自己生产的日常谷物，那么整个结论就错了，所以要否定这个假设。

A选项的意思，西非在引进大米和芋头之前，没有发展自己的日常谷物。

C没有关系。

清楚多了，还是自己多于A选项的英文意思没有理解透彻！谢谢