GMAT Critical Reasoning (CR) | Re: CR - Agr Societies

after i finished the passage, My rethinking was : the conclusion "discovering when rice and yams were introduced into western Africa would establish the earliest date at which agricultural societies could have arisen there" would not be true if Western Africa have another staple corps before getting the rice and rams. so my assumption was: Western Africa didn't plant other staple plants before receiving the Rice and Yams. but i couldn't find something similar to my rethinking among the
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