一台液控自动变速器使用辛普森齿系，最近给发动机作了检修和调整，车辆起步也正常。但换档

下列哪项是开放性气胸的突出表现（）。

A.纵隔扑动

B.血压骤降

C.极度呼吸困难

D.纵隔气肿

E.心跳骤停

How old is “old”?  The answer has changed over the years. Two hundred years ago，you were old at 35.At the beginning of the 20 th century，the average life span（平均寿命）was 45. In 1950，70-year-olds were really old.Today，a healthy 70-year-old is still thought young.

So，how old is old? The answer is one you’ve heard many times，from all kinds of people.“You are as old（or young）as you feel.”Your age simply tells you how many years you have lived.Your body tells you how well you’ve lived.

“Nobody grows old by living a number of years.” wrote a writer.“People grow old when they don’t have their ideals.”

People shouldn’t have the wrong ideas about aging.Sometimes，older minds can be as bright as young minds.Alice Brophy once said，“It makes me unhappy when people say，‘You look young for your age。’ What does that mean?

You know you can die old at 30 and live young at 80. ”

53.The meaning of the underlined word “aging” is “        ”

A．growing old      B. staying young    C. keeping healthy      D. feeling unhappy

54.Which question is NOT answered in the passage?

A. Can older people be as bright as young people?

B. What tells you how well you’ve lived?

C. How old is “old”?                     

D. what is the average life span today?

55.The passage is mainly about __________ .

A. the average life span                     B. aging

C. the 20th century                         D. older people and young people

数学家（）发明了函数的通用记法“f（x）”。