这题大全为何是c呀？？？

frankzhiwei

In 1987 sinusitis was the most common chronic medical condition in the United States, followed by arthritis and high blood pressure, in that order.
The incidence rates for both arthritis and high blood pressure increase with age, but the incidence rate for sinusitis is the same for people of all ages.
The average age of the United States population will increase between 1987 and 2000.
Which of the following conclusions can be most properly drawn about chronic medical conditions in the United States from the information given above?


(A) Sinusitis will be more common than either arthritis or high blood pressure in 2000.
(B) Arthritis will be the most common chronic medical condition in 2000.
(C) The average age of people suffering from sinusitis will increase between 1987 and 2000.
(D) Fewer people will suffer from sinusitis in 2000 than suffered from it in 1987.（C）
(E) A majority of the population will suffer from at least one of the medical conditions mentioned above by the year 2000.

问什么是C呢？感觉没提到呀！！！求解释，感谢！

sdcar2010

不懂 求解释

-- by 会员 lmf0801 (2010/12/2 20:39:56)

Read #2 post above.  Hopefully you can understand the math involved in answering this question.

lmf0801

不懂 求解释

maggie1121

好题，就把得s病的人看作整体人群就好理解了~~

sdcar2010

Note: same disease rate does not mean there will be the same quantity of sick people since the total number of the population might change.  Same disease rate simply means that the ratio between the sick among each age group is constant.

Having said that, this question is purely a math problem. Since the rate for sinusitis is independent of age, then the average age of patient having sinusitis is simply the average age of the whole population.

Suppose there are p people with the age of a, and within this age group there are s people who has sinusitis.  The age span is from a1 to an.

rate (of sinusitis) = s1/p1 = s2/p2 = s3/p3 = . . . = sn/pn

average age for the population = (a1*p1 + a2*p2 + a3*p3 + . . . + an*pn) / (p1 + p2 + p3 + . . . + pn)

average age for patients = (a1*s1 + a2*s2 + a3*s3 + . . . + an*sn) / (s1 + s2 + s3 + . . . + sn)
= (a1*p1*rate + a2*p2*rate + a3*p3*rate + . . . + an*pn*rate) / (p1*rate + p2*rate + p3*rate + . . . + pn*rate)
= [rate/rate][(a1*p1 + a2*p2 + a3*p3 + . . . + an*pn) / (p1 + p2 + p3 + . . . + pn)]
= (a1*p1 + a2*p2 + a3*p3 + . . . + an*pn) / (p1 + p2 + p3 + . . . + pn)
= average age for the population

paulinezhu

C.

A. Wrong, because there will be more people suffering from arthritis and high blood pressure in 2000. It is a result of the aging society and higher incidence rate of both letter.

B. Wrong. Both arthiritis and high blood pressure could be. In the paragraph it is not mentioned which of both is the common one.

C. Right. Same incidence rate, means same quantity of people would suffer from sinusitis, but people are getting older till 2000. So the patients would be older.

D. Wrong. The incidence rate remains the same for sinusitis.

E. Wrong. The passage did not mention this point.