一个逻辑题的一句话的讨论

Agricultural societies cannot exist without staple crops. Several food plants, such as kola and okra,are known to have been domesticated in westernAfrica, but they are all supplemental, not staple, foods. All the recordedstaple crops grown in westernAfrica were introduced from elsewhere, beginning, at some unknowndate, with rice and yams. Therefore,discovering when rice and yarns were introduced into westernAfrica would establish the earliest date at which agricultural societies could have arisen there.

Whichof the followingis an assumption on which the argument depends?

A.       People in westernAfrica did not develop staple crops thatthey stopped cultivating once rice and yams were introduced.

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

C.       Rice and yams were grown as staplecrops by the earliest agricultural societies outside of westernAfrica.

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

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

  

A的意思：（我觉得应该是）当rice and yams引进的时候，western Africa的人并没有生产出staple crops（也就说明了yams and rice是最早的staple crops，可以用来推测农耕社会的时间），但是，A选项的 that they stopped cultivating 就很奇怪，这是一个定语从句吧，但是，翻译起来就特别奇怪了：当y,r引进的时候，人们没有生产出他们停止种植的staple crops。