參考答案：

46. B.??? It weakens in one’s later years.

47. D.??? Some of them begin to decline when people are still young.

48. C.??? They function quite well even in old age.

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49. D.?? can put what they have learnt into more effective use.

50. A.??? find ways to slow down our mental decline.

答案解析：

46. B.??? It weakens in one’s later years.

解析：從原文第一段declining mental function is often seen as a problem of old age, “心智功能衰退經(jīng)常被看做是老年人的問題”，可以看出B是正確答案。

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47. D.??? Some of them begin to decline when people are still young.

解析：從原文第一段but certain aspects of brain function actually begin their decline in young adulthood , a new study suggests. “但是一項新的研究表明，大腦的某些功能在人們年輕的時候就開始衰退”，可以看出D是正確選項。

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48. C.??? They function quite well even in old age.

解析：從原文第五段中原句Most people’s minds function at a high level even in their later years, according to researcher Timothy Salthouse. “根據(jù)研究人員Timothy Salthouse的說法，大多數(shù)人的心智功能在他們晚年依然保持高水平工作”，可以看出C是正確選項。

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50. D.?? can put what they have learnt into more effective use.

解析：從原文第六段中原句but that the amount of knowledge one has, and the effectiveness of integrating it with one’s abilities, may increase throughout all of adulthood if there are no diseases. “但是只要一個人沒有疾病，那么這個人擁有的知識量，以及把知識和能力結(jié)合在一起的效率，可能會在整個成年時期增加”可以看出D是正確選項。

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50. A.??? find ways to slow down our mental decline.

解析：從原文倒數(shù)第二段中原句We gain insight in cognition changes, and may possibly discover ways to slow the rate of decline.“我們更加了解認(rèn)知變化，而且可能會發(fā)現(xiàn)減緩衰退的方法”，可以看出A是正確選項。