阅读理解。

     Like distance runners on a measured course, all of us will move through time in a roughly predictable

pattern.

     In the first stage of our lives, we develop and grow, reaching toward the top of physical

vitality(energy). 

     After we grow up,however, the body begins a process of gradually wearing out.

     A new awareness of physical fitness may help lengthen our years of health and vitality, yet nothing we  

do will work to stop the unavoidable force of aging.

     Most of the changes of aging take place deep inside the body. The lungs become less able to take in

oxygen. Powerful muscles gradually lose their strength. The heart loses power and pumps less blood.Bones grow easier to break.

     Finally, we meet a stress,a stress that is greater than our physical resistance. Often it is only a minor  

accident or chance infection(a disease eaused by virus), but this time,it brings life to an end.

     In 1932, a classic experiment nearly doubled the lifetime of rats, simply by cutting back the

calories(unit for measuring the energy value of food)in their diet.The reason for the effect was then

unknown.

     Today, at the university of California at Berkley, Dr.Paul Seagle has also greatly lengthened the

normal lifetime of rats. The result was achieved through a special protein(蛋白质)limited diet, which

had a great effect on the chemistry of the brain. Seagle showed that within the brain, specific chemicals

control many of the signals that influence aging. By changing that chemical balance, the clock of aging

can be reset.

     For the first time, the mystery(something that is difficult to understand or to explain)of why our age

is being seriously challenged. Scientists in many fields are now making striking and farreaching

discoveries.

     An average lifetime lasts 75 years, yet in each of us lies a potential for a longer life. If we could keep 

the vitality and resistance to disease that we have at age twenty, we would live for 80 years.

1. What does the underlined word"it" refer to?

A. Physical vitality.        

B. Stress.

C. Aging.                

D. Physical resistance.

2. The author believes the following EXCEPT that ________.

A. human's life pattern is predictable

B. physical fitness can't stop the force of aging

C. human's lifetime will last longer than 75 years

D. all of the changes of aging take place deep inside the body

3. How did Dr.Paul Seagle lengthen the lifetime of rats in his experiment?

A. By limiting the protein in their diet.

B. By cutting the calories in their diet.

C. By resetting their clock of aging.

D. By keeping their physical fitness.

4. What is probably the best title for the article?

A. Two Great Experiments

B. The Mystery of Human Life

C. Ways to Lengthen Human Life

D. The Breakthrough in the Study of Aging