46. 正确选项B。It weakens in one’s later years.
47. 正确选项D。Some of them begin to decline when people are still young.
48. 正确选项C。They function quite well even in old age.
49. 正确选项D。can put what they have learnt into more effective use.
50. 正确选项A。find ways to slow down our mental decline.