题目内容：
Section CDirections：There are 2 passages in this section．Each passage is followed by some questions or unfinished statements．For each of them there are four choices marked A ，B ，C and D ．You should decide on the best choice and mark the corresponding letter on Answer Sheet 2 with a single line through the centre．
Passage One
Questions 46 to 50 are based on the following passage．
Declining mental function is often seen as a problem of old age，but certain aspects of brain function actually begin their decline in young adulthood，a new study suggests．
The study,which followed more than 2,000 healthy adults between the ages of 18 and 60，found that certain mental functions-including measures of abstract reasoning．mental speed and puzzle-solving-started to dull as early as age 27．
Dips in memory,meanwhile，generally became apparent around age 37．
On the other hand．indicators of a person’s accumulated knowledge-like performance on tests of vocabulary and general knowledge-kept improving with age，according to findings published in the journal Neurobiology of Aging．
The results do not mean that young adults need to start worrying about their memories．Most people’s minds function at a high level even in their later years，according to researcher Timothy Salthouse．
“These patterns suggest that some types of mental flexibility decrease relatively early in adulthood，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，”Salthouse said in a news release．
The study included healthy,educated adults who took standard tests of memory,reasoning and perception at the outset and at some point over the next seven years．
The tests are designed to detect subtle(细微的)changes in mental function，and involve solving puzzles，recalling words and details from stories，and identifying patterns in collections of letters and symbols．
In general，Salthouse and his colleagues found，certain aspects of cognition(认知能力)generally started to decline in the late 20s to 30s．
The findings shed light on normal age—related changes in mental function，which could aid in understanding the process of dementia(痴呆)，according to the researchers．
“By following individuals over time，”Salthouse said，“we gain insight in cognition changes，and may possibly discover ways to slow the rate of decline．”
The researchers are currently analyzing the study participants’health and lifestyle to see which factors might influence age．related cognitive changes．
What is the common view of mental function? A．It varies from person to person．
B．It weakens in one’s later years．
C．It gradually expands with age．
D．It indicates one’s health condition
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