题目内容：

Exercise may help to safeguard the mind against depression(沮丧) through previously unknown effects on working muscles, according to a new study involving mice.

Mental health experts have long been aware that even mild, repeated stress can contribute to the development of depression and other mood disorders in animals and people. Scientists have also known that exercise seems to cushion against depression. But precisely how exercise, a physical activity can reduce someone’s risk for depression, a mood state, has been mysterious. So for the new study, researchers at the Karolinska Institute in Stockholm studied the brains and behavior of mice in a complicated and novel fashion.

We can’t ask mice if they are feeling cheerful or in low spirits. Instead, researchers have pictured certain behaviors that indicate depression in mice. If animals lose weight, stop seeking out a sugar solution when it’s available — because, probably, they no longer experience normal pleasures — or give up trying to escape from the cold-water zone just freeze in place, they are categorized as depressed. And in the new experiment, after five weeks of frequent but low-level stress, such as being lightly shocked, mice displayed exactly those behaviors. They became depressed.

The scientists could then have tested whether exercise blunts (延缓) the risk of developing depression after stress by having mice run first. But, frankly, from earlier research, they wanted to know how, so they bred pre-exercised mice. A wealth of earlier research by these scientists and others had shown that aerobic exercise, in both mice and people, increases the production within muscles of an enzyme (酶) called PGC-1alpha. The Karolinska scientists suspected（怀疑） that this enzyme somehow creates conditions within the body that protect the brain against depression. Then, the scientists exposed the animals, which without exercising, were in high levels of PGC-1alpha to five weeks of mild stress. The mice responded with slight symptoms of worry. But they did not develop depression. They continued to seek out sugar and fought to get out of the cold-water zone. Their high levels of PGC-1alpha appeared to make them depression-resistant（抵抗的）. Finally, to ensure that these findings are relevant to people, the researchers had a group of adult volunteers complete three weeks of frequent endurance training, consisting of 40 to 50 minutes of moderate cycling or jogging. The scientists conducted muscle biopsies (活体检查) before and after the program and found that by the end of the three weeks, the volunteers’ muscle cells contained substantially more PGC-1alpha than at the study’s start.

The finding of these results, in the simplest terms, is that “you reduce the risk of getting depression when you exercise,” said Maria Lindskog, a researcher at the Karolinska Institute.

1.The researchers at the Karolinska Institute in Stockholm conducted the new study hoping to know ________.

A. if exercise cushions against depression

B. what can lead to depression in animals and people

C. if stress can contribute to the development of depression

D. how exercise contributes to reducing someone’s risk for depression

2.We can infer from the new experiment conducted by researchers at the Karolinska Institute that mice are depressed except when ________.

A. they attempt to escape from the cold-water zone

B. they stop searching for the sugar water

C. they stand still in place

D. they can’t experience normal pleasures any longer

3.Researchers asked a group of adult volunteers to complete three weeks of frequent endurance training in order to ________.

A. know if exercise can help to safeguard the mind against depression

B. know if they can endure 40 to 50 minutes of moderate cycling or jogging

C. ensure they can lose weight after moderate cycling or jogging

D. confirm the findings above are also relevant to people

4.It can be concluded from the passage that ________.

A. the mice with high levels of PGC-1alpha are easier to develop depression

B. athletes are more likely to develop depression than ordinary people

C. the enzyme called PGC-1alpha helps to reduce depression

D. in the past mental health specialists didn’t know exercise could help reduce depression