破解 GMAT CR 題目的秘密武器｜CR 邏輯思路分析：平行推論

donzgmat
22小时前
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許多準備 GMAT 的同學，在面對 Critical Reasoning（邏輯推理）時，總是會遇到一個問題：題目明明看起來不難，但答案總是選錯。事實上，這往往不是因為你邏輯不好，而是你沒有學會「正確地解題」。

在 GMAT CR 中非常常見的邏輯工具——平行推論（Parallel Reasoning），特別是其中一種形式：「同為反向的平行推論」。

這個概念聽起來抽象，但只要透過實戰例題解析，你會發現它其實非常實用、好懂，而且是破解 CR 題目的秘密武器之一。

範例一：水鳥旋轉覓食的推理結構

我們從一題 GMATClub 的實際題目開始分析：

題目來源：When feeding, aquatic birds known as phalaropes…

When feeding, aquatic birds known as phalaropes often spin rapidly on the water’s surface, pecking for food during each revolution. To execute these spins, phalaropes kick one leg harder than the other. This action creates upwelling currents. Because plankton on which phalaropes feed typically occurs in greater quantities well below the surface, it is hypothesized that by spinning phalaropes gain access to food that would otherwise be beyond their reach.

Which of the following, if true, most strongly supports the hypothesis?

A. Phalaropes rarely feed while on land.

B. A given phalarope spins exclusively either to the right or to the left.

C. Phalaropes sometimes spin when they are not feeding.

D. Different phalaropes spin at somewhat different rates.

E. Phalaropes do not usually spin when food is abundant at the surface.

我們直接擷取結論: By spinning phalaropes gain access to food that would otherwise be beyond their reach.

而答案選項為: Phalaropes do not usually spin when food is abundant at the surface.

簡易因果關係如下: 題目: Phalaropes: Spinning >> Access to Food

答案 (Support): Phalaropes: Not Spinning >> No Need for Access to Food

這就是「同為反向的平行推論」：與其從「Spinning 帶來好處」去證明 spinning 的必要性，我們反過來觀察「沒有 spinning 時的條件」是否吻合。如果鳥兒在水面食物豐富時不會旋轉，那麼就更能支持牠們平常旋轉是為了吃深處的食物。

重點提醒：很多同學在做支持題時，會太依賴記憶的一些口訣或模板（例如「他因生他果」的套公式），但這樣容易落入出題者的陷阱。關鍵是：你是否真正看懂這個推論的結構？

範例二：高鹽度對抗生素效用的影響

讓我們再看一題具挑戰性的題目：

題目來源：Healthy lungs produce a natural antibiotic…

Healthy lungs produce a natural antibiotic that protects them from infection by routinely killing harmful bacteria on airway surfaces. People with cystic fibrosis, however, are unable to fight off such bacteria, even though their lungs produce normal amounts of the antibiotic. The fluid on airway surfaces in the lungs of people with cystic fibrosis has an abnormally high salt concentration; accordingly, scientists hypothesize that the high salt concentration is what makes the antibiotic ineffective.

Which of the following, if true, most strongly supports the scientist's hypothesis?

(A) When the salt concentration of the fluid on the airway surfaces of healthy people is raised artificially, the salt concentration soon returns to normal.

(B) A sample of the antibiotic was capable of killing bacteria in an environment with an unusually low concentration of salt.

(C) When lung tissue from people with cystic fibrosis is maintained in a solution with a normal salt concentration, the tissue can resist bacteria.

(D) Many lung infections can be treated by applying synthetic antibiotics to the airway surfaces.

(E) High salt concentrations have an antibiotic effect in many circumstances.

一樣擷取結論: The high salt concentration is what makes the antibiotic ineffective.

混淆答案

(B) A sample of the antibiotic was capable of killing bacteria in an environment with an unusually low concentration of salt.

(C) When lung tissue from people with cystic fibrosis is maintained in a solution with a normal salt concentration, the tissue can resist bacteria.

簡易因果關係如下:

題目: High Salt Concentration >> Antibiotic Ineffective

(B): Unusually Low Salt Concentration >> Antibiotic Capable

(C): Normal Salt Concentration >> Antibiotic Capable