CR 中的“无风不起浪”：快速排除无关选项

在 GMAT 的 CR 世界里，除了 GMAT 惯常的出题元素（例如 另有他因、倒果為因、動態平衡……）之外，有时掌握一些常识中的逻辑概念，就能快速排除无关选项。

这次要介绍的概念，可以用在 Explain 题之上，尤其是在矛盾事实涉及“两者的差异”或者需要找出“矛盾事件的起因”时。

练习题

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.

解题思路

文章中的矛盾事实是：“虽然参加喂食计划的成年海豚健康长寿，但它们后代却比较短命。”

这里的矛盾点在于“上一代”与“下一代”的差异，因此我们要找的答案，必须能解释这个差异。“用差异解释差异”原则，只有“差异”才能解释“差异”。

初步筛选

我们先找出选项中，哪些选项提到了“差异”：

• (A) Sharks …… are less common …… than …… (有差异)
• (B) Many of the adult dolphins …… are females that nurse …… (未提差异)
• (C) The fish given …… are the same types …… (明确指出“相同”，无法解释差异)
• (D) Many dolphins …… come to the beach only a few times a month. (未提差异)
• (E) Adult dolphins …… spend much less time teaching …… than do other …… (有差异)

初步筛选后剩下 (A) 和 (E)。

深入分析

接着，我们要看这个差异是否能精准对应到“上一代 v.s. 下一代”的寿命矛盾。

• 选项 (A) 分析：鲨鱼在哪个区域比较少，这跟“有没有参加喂食计划”没有直接关联。更重要的是，鲨鱼猎食海豚时，会分年纪吗？应该不会。既然老少通吃，那就无法解释为何只有下一代寿命较短。因此删除。
• 选项 (E) 分析：这个选项直接比较了“有参加喂食计划”和“没有参加”的成年海豚，它们在“教导后代捕食”这件事上花了不同的时间。这是一个明确的差异，并且直接影响了“后代”(offspring) 的生存能力，但不会影响到“成年海豚”(adult) 自己。这完美解释了为何只有后代的寿命较短。

因此，(E) 是正确答案。

CR 基本三步走

1. 阅读文章，找到核心概念与矛盾点。
2. 对应到相关的逻辑原则（如「无风不起浪」）来初步删除选项。
3. 最后在剩下的选项中，深入思考其合理性与对应关系。

（下一次预告：讲述CR概念中的「动态平衡」。）