问一道阅读里的逻辑题

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Increasingly, historians are blaming diseases imported from the Old World for the great disparity between the native population of America in 1492—new estimates of which jump as high as 100 million, or approximately one-sixth of the human race at that time—and the few million full-blooded Native Americans alive at the end of the nineteenth century. There is no doubt that chronic disease was an important factor in the sharp decline, and it is highly probable that the greatest killer was epidemic disease, especially as manifested in virgin-soil epidemics.
Virgin-soil epidemics are those in which the populations at risk have had no previous contact with the diseases that strike them and are therefore immunologically almost defenseless. That virgin-soil epidemics were important in American history is strongly indicated by evidence that a number of dangerous maladies—smallpox, measles, malaria, yellow fever, and undoubtedly several more—were unknown in the pre-Columbian New World. The effects of their sudden introduction are demonstrated in the early chronicles of America, which contain reports of horrible epidemics and steep population declines, confirmed in many cases by quantitative analyzes of Spanish tribute records and other sources. The evidence provided by the documents of British and French colonies is not as definitive because the conquerors of those areas did not establish permanent settlements and began to keep continuous records until the seventeenth century, by which time the worst epidemics had probably already taken place. Furthermore, the British tended to drive the native populations away, rather than to enslave them as the Spaniards did, so that the epidemics of British America occurred beyond the range of colonists' direct observation.
Even so, the surviving records of North America do contain references to deadly epidemics among the native population. In 1616—1619 an epidemic, possibly of pneumonic plague, swept coastal New England, killing as many as nine out of ten. During the 1630's smallpox, the disease most fatal to the Native American people, eliminated half the population of the Huron and Iroquois confederations. In the 1820's fever ruined the people of the Columbia River area, killing eight out of ten of them.
Unfortunately, the documentation of these and other epidemics is slight and frequently unreliable, and it is necessary to supplement what little we do know with evidence from recent epidemics among Native Americans. For example, in 1952 an outbreak of measles among the Native American inhabitants of Ungava Bay, Quebec, affected 99 percent of the population and killed 7 percent, even though some had the benefit of modern medicine. Cases such as this demonstrate that even diseases that are not normally fatal can have destroying consequences when they strike an immunologically defenseless community.

8. Which of the following, if newly discovered, would most seriously weaken the author’s argument concerning the importance of virgin-soil epidemics in the depopulation of Native Americans?

(A) Evidence setting the pre-Columbian population of the New World at only 80 million

(B) Spanish tribute records showing periodic population fluctuations

(C) Documents detailing sophisticated Native American medical procedures

(D) Fossils indicating Native American contact with smallpox prior to 1492

(E) Remains of French settlements dating back to the sixteenth century

答案：D

文章不用细读，读下框架知道作者讲什么就好了。
这道题说白了就是问：virgin-soil epidemics与depopulation of Native Americans的逻辑关系嘛。如果是因果关系，削弱就用同因异果或者第三因素导致因无法导致果；如果是果因关系，削弱就他因排除或者因果关系不存在。
按照答案的意思理解：virgin-soil epidemics与depopulation of Native Americans是因果关系，因为 Fossils indicating Native American contact with smallpox prior to 1492，所以当欧洲人来到美洲的时候smallpox就不是virgin-soil epidemics了，virgin-soil epidemics与depopulation of Native Americans的部分因果关系被瓦解了，也就是削弱了。

可从我的理解：作者是先发现depopulation of Native Americans然后找depopulation的原因呢？还是先发现virgin-soil epidemics，然后发现其结果是depopulation呢？
根据文章的第一段和作者的逻辑结构，显然是前者。
那么virgin-soil epidemics与depopulation of Native Americans的逻辑关系就应该是果因结构：先发现结果depopulation，之后找到导致结果的（唯一）原因是virgin-soil epidemics。
如果二者是果因结构，则削弱需要：他因排除或者因果关系不存在
选项B不就是他因排除吗？Spanish tribute records showing periodic population fluctuations，说明是periodic population fluctuations导致了depopulation of Native Americans。

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