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 caused 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 we age is being seriously challenged. Scientists in many fields are now making striking and far-reaching 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 twenty, we would live for 800 years.

　　1. What does the underlined word “it”refer to?

　　A. Physical vitality.B. Stress.C. Aging.D. Physical resistance.

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