在我們的GRE閱讀體系中，通過句間關係來對下文進行預判，並且理清文章的邏輯脈絡。而對GRE閱讀來說，句間關係只有兩種，順承和轉折。
轉折關係有如下三個標誌：
①句子中出現了but,yet, however, nevertheless這四個轉折連詞；
②句子中出現了表示否定態度的詞：oppose, criticize, overestimate等等；
③兩句話討論的對象發生了變化。
除了這三種，句子之間的關係都是順承。那麼，很多同學會有疑問，by contrast的意思是「與之相比的是」，那麼當文中有by contrast出現時，是否也意味著轉折的出現呢？讓我們來看如下的幾個真題例句：
1.This last criterion is more easily met in dating marine sediments, because dating of only a small number of layers in a marine sequence allows the age of other layers to be estimated fairly reliably by extrapolation and interpolation. By contrast, because sedimentation is much less continuous in continental regions, estimating the age of a continental bed from the known ages of beds above and below is more risky.
【譯】上一個標準對於測定海洋沉積物的年齡來說更合適，因為僅測定少量的海洋層可以讓其他層的年齡通過推測和插補來被相當靠譜地估計。與之相比的是，在大陸地區由於沉積物不是那麼的連續，所以從已知陸床年齡來推測另外一個是更有風險的。
前面是測定海洋沉積物的年齡，方法可行；後面是測定大陸沉積物的年齡，方法不可行。對象變了，文意發生轉折。
2. Early models of the geography of the metropolis were unicellular: that is, they assumed that the entire urban district would normally be dominated by a single central district, around which the various economic functions of the community would be focused. This central business district (CBD) is the source of so-called high-order goods and services, which can most efficiently be provided from a central location rather than from numerous widely dispersed locations. Thus, retailers of infrequently and irregularly purchased goods, such as fur coats, jewelry, and antique furniture, and specialized service outlets, such as theaters, advertising agencies, law firms, and government agencies, will generally be found in the CBD. By contrast,less costly, more frequently demanded goods, such as groceries and housewares, and low-order services, such as shoe repair and hairdressing, will be available at many small, widely scattered outlets throughout the metropolis.
之前是討論高價產品和服務適合於CBD，by contrast之後討論的對象變了，低價大眾快消品不適合CBD這種集中的區域，適合於mall。
綜上所述，當by contrast出現時，同時也意味著討論的對象發生了變化，這也與我們之前說的句間轉折標誌是一致的。所以針對by contrast這個詞組，我們不需要刻意記憶它的功能。
最後還有一個配套練習哦，回復「對比」，查看練習答案
練習：
Large-scale enterprises tend to operate more comfortably in stable and secure circumstances, and their managerial bureaucracies tend to promote the status quo and resist the threat implicit in change. Moreover, in some cases, industrial giants faced with little or no competition seek to avoid the capital loss resulting from obsolescence by deliberately obstructing technological progress. By contrast, small firms undeterred by large investments in plant and capital equipment often aggressively pursue new techniques and new products, investing in innovation in order to expand their market shares.
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