GWD19的一道题

Q40:请问这题为什么选A啊？

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:

这个assumption题是一个典型的defender，也就是排除一个削弱原文推理的事件。

如果把选型A取非：People in western Africa develop staple crops that they stopped cultivating once rice and yams were introduced，那就说明就算知道什么时候引入rice and yams也不能确定agricultural societies建立的最早日期，因为有比rice and yams更早的staple crops存在，而这些未知的crops才是能确定agricultural societies建立时间的关键。

A排除了削弱原文推理的事件，所以正确。

但是题干里已经说了All the recorded staple crops grown in western Africa were introduced from elsewhere，那就说明没有staple crops 是People in western Africa developed 的啊？

要不就是我对recorded理解有误？

请指教

A选项中出现一个短语“stopped cultivating ”，可能有这层意思：如果存在那些staple crops ，并且已经stopped cultivating 了的话，可能就没有记载了，也就没有recording了，而且，就all recording来说，也不一定能够包括所有可能存在过的staple crops ，有些可能没有记录了！

Agricultural societies cannot exist without staple crops. 

discovering when rice and yams were introduced into western Africa would establish the earliest date at which agricultural societies could have arisen there.

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

No prior staple crop eve developed in Africa, or the establishment of its agricultural society would be earlier , since agricultural societies cannot exist without staple crops