为什么OG196得C可以是答案而OG137得A不能使答案得？

OG10-137

 A study of marital relationships in which one partner's sleeping and waking cycles differ from those of the other partner reveals that such couples share fewer activities with each other and have more violent arguments than do couples in a relationship in which both partners follow the same sleeping and waking patterns. Thus, mismatched sleeping and waking cycles can seriously jeopardize a marriage. Which of the following, if true, most seriously weakens the argument above?

(A) Married couples in which both spouses follow the same sleeping and waking patterns also occasionally have arguments that can jeopardize the couple's marriage.

(B) The sleeping and waking cycles of individuals tend to vary from season to season.

(C) The individuals who have sleeping and waking cycles that differ significantly from those of their spouses tend to argue little with colleagues at work.

(D) People in unhappy marriages have been found to express hostility by adopting a different sleeping and waking cycle from that of their spouses.
                D
                

(E) According to a recent study, most people's sleeping and waking cycles can be controlled and modified easily.
                

答案虽然是D，但是A其实也有weaken得作用，只不过OG解释里说The argument does not depend on there being only one cause of marital problems, so choice A is incorrect。也就是说A只是唯一个导致婚姻失败的原因，所以A不对。下面再来看看OG10-196

196.An unusually severe winter occurred in Europe after the continent was blanketed by a blue haze resulting from the eruption of the Laki Volcano in the Europeans republic of Iceland in the summer of 1984. Thus, it is evident that major eruptions cause the atmosphere to become cooler than it would be otherwise.

Which of the following statements, if true, most seriously weakens the argument above?

(A) The cooling effect triggered by volcanic eruptions in 1985 was counteracted by an unusual warming of Pacific waters.

(B) There is a strong statistical link between volcanic eruptions and the severity of the rainy season in India.

(C) A few months after EI Chichon’s large eruption in April 1982, air temperatures throughout the region remained higher than expected, given the long-term weather trends.

(D) The climatic effects of major volcanic eruptions can temporality mask the general warming trend resulting from an excess of carbon dioxide in the atmosphere.

(E) Three months after an early springtime eruption in South America during the late 19th century, sea surface temperatures near the coast began to fall.
                C
                

答案是C不也就用了一个反例来weaken了argument得么？为什么196得C可以是答案而137得A不能使答案得？