请教GWD9-40(也就是GWD24的7-40）

snoopy121

题的内容是
gwd9-40

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?



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



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



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



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



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

这道题我这份资料的答案是A，但是我看到论坛中有人说是B.
我想请问一下那个是正确的答案。
先谢谢了

海贝520

斗胆来说一下，以后如果有人搜这道题的话希望能看见，虽然提问者是09年问的。。。不指望能给你解答了。
这题的基本类型我归为plan-goal类
plan是用 rice和ram的时间来推导出农业社会形成的时间
goal就是农业社会形成的时间。
问plan-goal类型的assumption的话，我自己这么区分：若要plan成功，就要排除plan中的各种漏洞/若要plan不成功，就要说plan有漏洞（or plan不论直接间接方式都不可能成功）



所以A的本意是：并不是因为rice和yam引进后就不生产原来的主食了
取非：rice和yam引进后就不生产之前的主食了。削弱
B：if domesticated是插入语，先忽略，意思就是西非本来也没有可以当做主食的本地植物。
可以看出B是无关选项。
剩下的都很好排除