174. 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. (E) A majority of the population will suffer from at least one of the medical conditions mentioned above by the year 2000.

Given that the incidence rate for sinusitis is the same for people of all ages, and that the average age of the population will increase, if follows that the average age of people suffering from sinusitis will increase. Therefore, C is the best answer.

什么道理人口的平均年龄会增加？牛人请帮我解释一下

好拗口的解释，不知道你看不看的懂