Q40:

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?

staple food crops.

这里我想问一下， A 选项rice,yam之前， 西非的人们没有develop staple food, 可是如果取非，好像有点反对前提的样子，因为文中已经提到了“All the recorded staple crops grown in western Africa were introduced from elsewhere, beginning, at some unknown date, with rice and yams”。

我对这一点很疑惑， 可否请N教？

谢谢！

"All the recorded staple crops grown in western Africa were introduced from elsewhere, beginning, at some unknown date, with rice and yams."

The keyword is "recorded". It could be the case that some staple crops grown there were somehow not recorded.

BTW, please always post the complete answer choices when asking questions. And, since this question has been discussed before, it's usually better to simply follow up in the original thread.