参考答案：

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.

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：从原文第一段declining mental function is often seen as a problem of old age, “心智功能衰退经常被看做是老年人的问题”，可以看出B是正确答案。

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

解析：从原文第一段but certain aspects of brain function actually begin their decline in young adulthood , a new study suggests. “但是一项新的研究表明，大脑的某些功能在人们年轻的时候就开始衰退”，可以看出D是正确选项。

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

解析：从原文第五段中原句Most people’s minds function at a high level even in their later years, according to researcher Timothy Salthouse. “根据研究人员Timothy Salthouse的说法，大多数人的心智功能在他们晚年依然保持高水平工作”，可以看出C是正确选项。

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

解析：从原文第六段中原句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. “但是只要一个人没有疾病，那么这个人拥有的知识量，以及把知识和能力结合在一起的效率，可能会在整个成年时期增加”可以看出D是正确选项。

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

解析：从原文倒数第二段中原句We gain insight in cognition changes, and may possibly discover ways to slow the rate of decline.“我们更加了解认知变化，而且可能会发现减缓衰退的方法”，可以看出A是正确选项。