CR 中的「無風不起浪」原則，讓你快速排除無關選項

在 GMAT 的 CR 世界裡，除了GMAT慣常的出題元素（例如另有他因、倒果為因、動態平衡…）之外，掌握如「GMAT CR 無風不起浪原則」這類常理中的邏輯概念，有時能幫助你快速排除無關選項。

這次要介紹的概念，可以用在 Explain 題之上，尤其是當矛盾事實在講「兩者差異」或者找「矛盾事件起因」的時候。

掌握 GMAT CR 無風不起浪原則

練習題

A popular beach has long had a dolphin feeding program in which fish are given to dolphins several times a day; many dolphins get as much as half of their food each day there. Although dolphins that first benefit from the program as adults are healthy and long-lived, their offspring have a lower life expectancy than offspring of dolphins that feed exclusively in the wild.

Which of the following, if true, most helps to explain the lower life expectancy of offspring of dolphins feeding at the beach compared to other young dolphins?

1. Sharks that prey on dolphins are less common in the open seas off the beach than in many other areas of the open seas where dolphins congregate.
2. Many of the adult dolphins that feed at the beach are females that nurse their offspring there.
3. The fish given to the dolphins at the beach are the same types of fish that dolphins typically catch in the wild.
4. Many dolphins that feed at the beach with their offspring come to the beach only a few times a month.
5. Adult dolphins that feed at the beach spend much less time teaching their offspring how to catch fish in the wild than do other adult dolphins.

解題思路

文章中的矛盾事實是：「雖然參加餵食計畫的成年海豚健康長壽，但牠們的後代卻比較短命。」

這裡的矛盾點在於「上一代」與「下一代」的差異，因此我們要找的答案，必須要能解釋這個差異。依據「無風不起浪」原則，只有「差異」才能解釋「差異」。

初步篩選

我們先找出選項中，哪些選項提到了「差異」：

• (A) Sharks … are less common … than … (有差異)
• (B) Many of the adult dolphins … are females that nurse … (未提差異)
• (C) The fish given … are the same types … (明確指出「相同」，無法解釋差異)
• (D) Many dolphins … come to the beach only a few times a month. (未提差異)
• (E) Adult dolphins … spend much less time teaching … than do other … (有差異)

初步篩選後剩下 (A) 和 (E)。

深入分析

接著，我們要看這個差異是否能精準對應到「上一代 v.s. 下一代」的壽命矛盾。

• 選項 (A) 分析：鯊魚在哪個區域比較少，這跟「有沒有參加餵食計畫」沒有直接關聯。更重要的是，鯊魚獵食海豚時，會分年紀嗎？應該不會。既然老少通吃，那就無法解釋為何只有下一代壽命較短。因此刪去。
• 選項 (E) 分析：這個選項直接比較了「有參加餵食計畫」和「沒有參加」的成年海豚，牠們在「教導後代捕食」這件事上花了不同的時間。這是一個明確的差異，並且直接影響了「後代」(offspring) 的生存能力，但不會影響到「成年海豚」(adult) 自己。這完美解釋了為何只有後代的壽命較短。

因此，(E) 是正確答案。

CR 基本三步走

1. 閱讀文章，找到核心概念與矛盾點。
2. 對應到相關的邏輯原則（如「無風不起浪」）來初步刪去選項。
3. 最後在剩下的選項中，深入思考其合理性與對應關係。

（下一次預告：講述CR概念中的「動態平衡」。）