2. 外语
  3. Upon maturity, monarch butterflies travel hundreds of miles from their places of origin and lay their eggs on milkweed. The cate

Upon maturity, monarch butterflies travel hundreds of miles from their places of origin and lay their eggs on milkweed. The cate

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问题     Upon maturity, monarch butterflies travel hundreds of miles from their places of origin and lay their eggs on milkweed. The caterpillars that emerge feed on milkweed and absorb the glycosides in milkweed sap. The specific glycosides present in milkweed differ from region to region within the monarch butterfly’s range. Since mature butterflies retain the glycosides, the glycosides in a mature monarch butterfly could be used to determine its place of origin.
    Which of the following, if true, most strengthens the argument?
选项 A、Mature monarch butterflies do not feed on parts of milkweed that contain glycosides.
B、The glycosides in milkweed sap are slightly toxic to caterpillars of other species.
C、The vast majority of the monarch butterflies that are laying eggs in a given region will have traveled there from a single region.
D、There are substances other than glycosides in milkweed sap that accumulate in a monarch caterpillar and are retained in the body of the mature butterfly.
E、There are certain glycosides that are found in the sap of all milkweeds, no matter where they grow within the monarch butterfly’s range.
答案A
解析 既然成熟蝴蝶不会再吃其他能够让其出现配糖体的植物。因此蝴蝶身上的配糖体只能是其幼虫时期所得到的。A项属于排除他因的加强，正确。B项其他物种的幼虫属于无关选项。C项成熟蝴蝶从哪里出发和问题无关。D项本题只讨论配糖体能否用于确定蝴蝶起源地，至于蝴蝶体内是否有其他物质，和本题无关。E项蝴蝶体内只要有配糖体，就可以确定其起源地。至于是不是所有植物都有配糖体，和本题无关。
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