GWD 9-20 Which of the following most logically completes the reasoning? Either food scarcity or excessive hunting can threaten a population of animals. If the group faces food scarcity, individuals in the group will reach reproductive maturity later than otherwise. If the group faces excessive hunting, individuals that reach reproductivematurity earlier will come to predominate. Therefore, it should be possible to determine whether prehistoric mastodons became extinct because of food scarcity or human hunting, since there are fossilized mastodon remains from both before and after mastodon populations declined, and ______. A. there are more fossilized mastodon remains from the period before mastodon populations began to decline than from after that period B. the average age at which mastodons from a given period reached reproductive maturity can be established from their fossilized remains C. it can be accurately estimated from fossilized remains when mastodons became extinct D. it is not known when humans first began hunting mastodons E. climate changes may have gradually reduced the food available to mastodons

答案是E ，可是，我还是没有彻底搞懂，请大家再帮忙理理，为什么动物的遗迹都在数量下降的前后能说明是由于食物缺乏造成的呢？还有，如果是由于hunting造成的，那遗迹应该是怎么样的？ 谢谢先啦

答案为B

原文知两个关系：如果FACE FOOD SCARCITY，则慢到reproductive maturity 。如果FACE EXCESSIVE HUNTING，则早到reproductive maturity （早到reproductive maturity 的占大多数）。

所以作者认为有可能确定M是由于FOOD SCARCITY灭绝，还是EXCESSIVE HUNTING灭绝。理由是有M数量减少前后的化石。另外，B说可从化石知道到达reproductive maturity 的平均年龄。因为如果减少前的reproductive maturity 年龄大于减少后的年龄，则原因是EXCCESIVE HUNTING（根据第二个关系）。如果小于，则是FOOD SCARCITY（第一个关系）