所謂的“套娃”指的是文章會陳述多個物件，彼此之間形成直接或間接的形成環環相扣的因果關係。由於牽涉的物件較多，讀這類文章的關鍵就在於從文字當中提煉出物件之間的正負關聯，從而更直觀地反映出它們之間的聯絡。
比如下面這段文字：
Geese can often be seen grazing in coastal salt marshes. Unfortunately, their intense grazing removes the grassy covering, exposing marsh sediment; this increases evaporation, which in turn increases salt concentration in marsh sediments. Because of this increased concentration, regrowth of plants is minimal, leading to increased erosion, which leads to a decrease in the fertile topsoil, leading to even less regrowth. In time, the salt marsh becomes a mudflat.
鵝群在鹽沼中高強度的覓食行為會剝離表層植被，暴露出沼澤沉積物，這會導致水分蒸發增多，提高沉積物鹽濃度，繼而引發植被重新生長變少，從而導致水土流失增加，表面肥沃的土壤減少，而這又進一步減少了植被重新生長的數量。
好傢伙，鵝群的覓食就像多米諾骨牌引發了一連串的連鎖反應，最後形成了一個self-perpetuating的惡性迴圈
geese grazing↑— grassy covering↓— evaporation↑— salt concentration↑— plant regrowth↓— erosion↑— fertile topsoil↓— plant regrowth↓↓
把文字轉換為邏輯鏈之後，我們就能很容易得知因素之間的正負相關性，有利於理清做題思路。
比如下面這道機經真題：
In 1995, after an absence of nearly 70 years, wolves were reintroduced into Yellowstone National Park. During the wolf-free era, heavy browsing of aspen trees by elk populations spelled doom not only for trees themselves but for a host of other creatures dependent on them, such as beavers, whose population in Yellowstone crashed after wolves were removed. Without beavers to create ponds, wetland ecosystems—aquatic plants, amphibians, birds—were devastated. When wolves returned, grazers and browsers resumed normal patterns of behaviors, preferring safer, open areas over the dense cover and streamsides where carnivores can lurk. Keeping elk wary and on the move, wolves gave aspen and other young trees the opportunity to grow and become reestablished.
沒有狼的時期，麋鹿elk大量啃食山楊樹aspen trees，導致樹木大量減少，同時依賴山楊樹生存的海狸beaver等生物也大量減少，一旦失去海狸建造池塘，那麼包括水生植物、兩棲動物和鳥類在內的溼地生態系統便遭到了破壞；重新引入狼可以讓麋鹿四處遷徙，從而讓樹木有機會恢復。
wolf↓— elk browsing↑— aspen trees↓— beavers↓— ponds↓— wetland↓(aquatic plants↓ amphibians↓ birds↓)
❓1. The passage asserts which of the following about beaver populations in Yellowstone?
A. They have rebounded since the reintroduction of wolves.
B. They were adversely affected by the feeding habits of elk population.
C. They increased during the period when wolves disappeared from the park.
D. They have historically had an adverse effect on the park’s wetland ecosystems.
E. They are essential to the health of the park’s aspen trees.
題幹：關於beaver的數量，文章表達了以下哪個觀點？
❌A. 重新引入狼之後，beaver數量已經回升。錯誤，“have rebounded”表示已經發生的狀況，但原文並未明確指出beaver數量確實已經回升。
B. beaver數量過去遭到elk覓食行為的負面影響。正確，1995年之前沒有狼的時期，elk啃食樹葉導致beaver數量減少，對其造成負面影響。
❌C. 狼消失之後，beaver數量增加了。錯誤，beaver消失了（removed）而非增加了。
❌D. beaver數量過去（historically）對溼地生態系統有負面影響。錯誤，beaver對於溼地有利。
❌E. beaver對於山楊樹aspen trees的健康不可或缺。錯誤，aspen不需要依賴beaver。
❓2. The author would most likely agree with which of the following claims about the reintroduction of wolves to Yellowstone?
A. It indirectly harmed some of the park’s amphibian habitats
B. It reduced the number of elk feeding along streamsides
C. It led to greater species diversity among the park’s grazers and browsers.
D. It significantly increased competition for food among the park’s carnivores.
E. It fostered the resurgence of tree species that once flourished in the park’s open areas.
題幹：關於重新引入狼群，作者最有可能同意哪個說法？
❌A. 它間接損害了某些兩棲類動物棲息地。錯誤，透過邏輯鏈可知，狼群數量和溼地生態系統呈正相關，所以並不會損害兩棲類動物棲息地。
B. 它減少了在河邊（streamsides）覓食的麋鹿數量。正確，植被茂密的地方（dense cover）以及河邊都可能有肉食動物潛伏，狼迴歸之後麋鹿更喜歡在開闊的地區（open areas）活動。
❌C. 它導致草食動物物種多樣性的提高。錯誤，原文並未提及狼和草食動物物種數量之間的聯絡。
❌D. 它極大地增加了肉食動物對食物的競爭。錯誤，原文未提到狼與其他肉食動物的競爭。
❌E. 它導致曾經在開闊地區（open areas）繁茂的樹木復甦。錯誤，重新引入狼群后，麋鹿更喜歡在開闊地區活動，這裡的樹木很難復甦。
題目中出現多個物件之間因果關係時，我們需要梳理物件之間邏輯鏈及其正負相關性，當其中某個變數發生改變時，便可以很容易判斷其他因素改變的方向。
接下來給大家留一道作業，歡迎同學們積極留言，把答案和思路寫在評論區，我們會在下午6：00前公佈置頂正確答案~
In explaining his opposition to efforts aimed at stabilizing moose populations and at suppressing fires in northern forests, Pastor points out that moose, by feeding on hardwood trees, contribute to a characteristic process whereby hardwood trees are eventually overtaken and shaded out, as conifers such as spruces block sunlight. A decline in hardwood trees leads to fewer nitrogen-rich, quick-rotting leaves on the forest floor, which is increasingly carpeted with slower-rotting conifer needles. Hardwoods require more nitrogen for growth than do conifers, so the decline in soil fertility in areas where moose feed reinforces the advance of conifers. But conifers cannot dominate the forest for long, because mature conifer forests are more combustible than are hardwood forests. Fire combined with high winds, attacks of spruce budworm, or even a greatly diminished nitrogen supply will destroy expanses of spruces, allowing hardwoods to recolonize in the nutrient-rich ash. Such oscillations in forest composition and consequently in moose population are, Pastor believes, essential to the integrity and functioning of northern forest ecosystems.
❓The passage suggests which of the following about conifers?
A. They enrich the soil with nitrogen more than do hardwood.
B. They are relatively long-lived compared with hardwoods.
C. They require nutrient-rich ash in order to recolonize.
D. They are adversely affected by increases in moose populations.
E. They are adversely affected by extremely low levels of nitrogen.
