2016年1月8日 星期五

[CR] CR思路解析:「無風不起浪」(1)

原文發表於Dustin部落格:http://dustingmat.blogspot.tw/2016/01/cr-1.html

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,有時候,掌握某些常理中的邏輯概念,就能快速排除無關選項。

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。
這個概念就是:無風不起浪--「不同解釋不同」「改變帶來改變」

請各位先做這一題:


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?

A. 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.

B. Many of the adult dolphins that feed at the beach are females that nurse their offspring there.

C. The fish given to the dolphins at the beach are the same types of fish that dolphins typically catch in the wild.

D. Many dolphins that feed at the beach with their offspring come to the beach only a few times a month.

E. 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.

這一題的文章中說了什麼矛盾事實?
「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. 」

兩個矛盾事實是:
1. 從餵食計劃中得到食物的海豚雖然健康長壽
2. 他們的後代卻比較短命

請注意:這裡的矛盾事實在講「兩者差異」
要去解釋的就是「上一代與下一代為何有這種差異?」
這時候,請記住「無風不起浪」原則。

舉個例子好了,當今天我們看到「小明考100分,小華考30分」
身為小華的爸媽,想要知道原因,我們會怎麼想?

「因為小明跟小華下課都有去補習班」
這是原因嗎?當然不是。因為兩個人都做一樣的事,那怎麼最後還是我家小華成績比較爛?

「因為小明很認真念書」
乍看之下很像是原因對吧?但搞不好「小華也跟小明一樣認真!」「小明很認真念書」不代表「小華不認真念書」。

再舉一個DS題當例子:

Is x>y?

(1) x = 5
(2) y = 3

請問答案是多少?很明顯是C吧。
為什麼不是A or B?因為只確定條件一的話,y的值會亂跳。
只確定條件二的話,x的值會亂跳。
其實就是以上這個概念。要點出「差異」,才能解釋「差異」。

另外,如果解釋題講的是「某個矛盾事件找原因」,
那麼,這時候我們要找的原因必須要是「事件、改變」。
舉個例子,若要解釋的事件是「為什麼某地會有滔天巨浪?」

我們會怎麼解釋?
「因為海水的性質密度高」
這能解釋嗎?顯然不能。因為這是不動的性質,而不是導致事情的原因。

「因為有風、有地震、有惡劣的天氣狀況...」
這才是合理的原因吧。請記住,事件才會導致事件,改變才會帶來改變。
--「無風不起浪」。


回到我們剛剛那題的選項,既然知道是要解釋文章裡面的
「成年海豚與後代為何有如此差異」,那我們就要鎖定有點出差異的選項,沒有點出明顯差異的選項就要排除。

A. 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.(有點出明顯差異)

B. Many of the adult dolphins that feed at the beach are females that nurse their offspring there.
(沒有點出明顯差異)。

C. The fish given to the dolphins at the beach are the same types of fish that dolphins typically catch in the wild.
(沒有點出明顯差異)

D. Many dolphins that feed at the beach with their offspring come to the beach only a few times a month.
(沒有點出明顯差異)

E. 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.
(有明顯差異)

初步篩選之後剩下這兩個:

A. 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.

E. 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.

再來,要看的就是「這個差異對應到的是否為矛盾事實中點出的差異?」
矛盾事實講的是「參加餵食計劃的上一代壽命長,下一代壽命短」
這裡要分別對應到「有無參加餵食計劃」跟「有沒有辦法只影響其中一代不影響另一代」
若是兩邊都會影響或都不會影響的,那就依據「無風不起浪原則」刪去。

A. 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.

「生活在不同的區域」真的等於「有無參加餵食計劃?」嗎?顯然不能推得。
再者,「鯊魚獵食海豚」會只吃年輕海豚不吃老海豚?選項中只寫prey on dolphin,代表他是一視同仁,吃老的也吃小的。這時,依據「無風不起浪原則」,相同點不能拿來解釋差異
既然鯊魚老少通吃,那為何少的還是活得比老得短?
因此,將A選項刪去。


E. 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.

選項中直接點出了比較對象是「有沒有參加餵食計劃(feed at the beach)」也點出了差異事實「less time teaching their offspring...」用差異來解釋差異。也直接點出了上一代跟下一代(adult / offspring)。因此,其為正確答案。

所以你會發現,做CR花一分鐘到一分半來做並不困難。

閱讀文章找到核心概念與套路->
對應到該套路相應的邏輯基本概念來初步刪去選項->
最後留下的選項再去深入故事思考合理性

以上,就是CR題的基本三步走。