GWD-9-40，为何不选B？望指教，多谢

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?

1. People in western Africa did not develop staple crops that they stopped cultivating once rice and yams were introduced.

2. There are no plants native to western Africa that, if domesticated, could serve as staple food crops.

3. Rice and yams were grown as staple crops by the earliest agricultural societies outside of western Africa.

4. Kola and okra are better suited to growing conditions in western Africa than domesticated rice and yams are.

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

Answer is A

题目说Agricultural的标志是有staple crops，所以文章说鉴于rice 和yams是最早的，所以earliest date就追溯到rice 和yams

那里就有一个assumption ，因为现有的food 都是supplemental，所以在 rice 跟 yams之前就没有stable crops。

A 不存在不能考证的stable crops。

B 逻辑的内容讲的主要是staple跟agricultural的历史渊源，并不是讲staple food的问题。无关

还是不太明白，我明白A是对的，但B，no plants could serve as staple food crops，后来有了rice and yams，不久可以知道是从何时起的吗？

如果有另外一种外来食物，在很久之前一直是staple food，直到引入rice and yams。那么就不能定论在rice and yams开始

注意，B中间有个native to

还是不太明白，请再详细解释一下b为什么不对~THX

我觉得B是无关选项。有没有这种植物和当地人去不去domesticate them 似乎没有必然联系。