再问GWD-9-40

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?

"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.