题目内容：

Can exercise during childhood protect you against memory loss many decades later?Exercise early in life seems to have lifelong benefits for the brain,in rats at least.

“This is an animal study,but it shows that physical activity at a young age is very important—not just for physical development,but for the whole lifelong track of cognitive（认知的）development during ageing,”says Martin Wojtowicz of the University of Toronto,Canada.“In humans,it may delay the appearance of Alzheimer's symptoms（阿茨海默氏症）,possibly to the point of preventing them.”

Wojtowicz’s team divided 80 young male rats into two equal groups,and placed running wheels in the cages of one group for a period of six weeks.Around four months later—when the rats had reached middle age—the team taught all the rats to connect an electric shock with being in a specific.When placed in the box,they froze with fear.

Two weeks later,the team tested the rats in three situations: exactly the same box in the same room,the same box with the room arranged differently,and a completely different box in a different room.

The rats without access to a running wheel when they were young now froze the same percentage of times in each of these situations,suggesting they couldn't remember which one was dangerous.But those that had been able to run in their youth froze 40 to 50 percent less in both changed box settings.

"The results suggest the amount of physical activity when we're young,at least for rats,has influence on brain and cognitive health—in the form of better memories—when we're older,"says Arthur Kramer of Northeastern University in Boston,who has found that,in humans,exercise promotes the growth of new brain cells.

1.The study shows that_____.

A. physical activity is important for physical health

B. using the running wheels is of benefit to the rats’ growth

C. physical activity can prevent human’s Alzheimer's symptoms

D. the more exercise a rat has when young,the better memory it will possess when older

2.How are Paragraph 3 and 4 mainly developed?

A. By analyzing causes.    B. By giving an example.

C. By describing the process.    D. By showing differences.

3.What does the underlined word "it" in Paragraph 2 refer to?

A. Exercise.    B. Development.

C. Benefit.    D. Study.

4.What is the author's attitude towards the animal study?

A. Negative.    B. Objective.

C. Critical.    D. Doubtful.