Living in a city is a well-known risk factor for developing a mental disorder，while living close to nature is largely beneficial for mental health and the brain.A central brain region，the amygdala（大腦杏仁核）involved in stress processing，has been shown to be less activated during stress in people living in rural areas，compared to those who live in cities，hinting at the potential benefits of nature. "But so far the hen-and-egg problem could not be resolved，namely whether nature actually caused the effects in the brain or whether the particular individuals chose to live in rural or urban regions，" says Sonja Sudimac.
   To achieve causal evidence，the researchers from the group examined brain activity in regions involved in stress processing in 63 healthy volunteers before and after a one-hour walk in Grunewald forest or a shopping street with traffic in Berlin The results revealed that activity in the amygdala decreased after the walk in nature suggesting that nature elicits beneficial effects on brain regions related to stress.
   "The results support the previously assumed positive relationship between nature and brain health，but this is the first study to prove the causal link.Interestingly，the brain activity after the urban walk in these regions remained stable and showed no increases，which argues against a commonly held view that urban exposure causes additional stress，" explains Simone Kühn，head of the group.
   The authors show that nature has a positive impact on brain regions involved in stress processing and that it can already be observed after a one-hour walk.This contributes to the understanding of how our physical living environment affects brain and mental health.Even a short exposure to nature decreases amygdala activity，suggesting that a walk in nature could serve as a preventive measure against developing mental health problems and reducing the potentially disadvantageous impact of the city on the brain.
   In order to investigate beneficial effects of nature in different populations and age groups，the researchers are currently working on a study examining how one-hour walk in natural versus urban environments impacts stress in mothers and their babies.

（1）What does the hen-and-egg problem mean by Sonja Sudimac？

A

A

A．It's hard to clarify the impact of living environment on mental health.
B．People in cities tend to have a higher risk of mental problems.
C．It's unnecessary to work out the complex issue of stress and health.
D．Living in rural areas actually affects brain activity.
（2）How did researchers collect evidence for the study？

D

D

A．By quoting authoritative experts.
B．By interviewing healthy volunteers.
C．By researching on a previous theory.
D．By comparing volunteers' amygdala activities.
（3）What contribution did Simone Kühn and his team mainly make？

A

A

A．Confirming the assumption of nature's benefits to brain health.
B．Supporting a commonly held view of urban exposure.
C．Revealing the link between age and mental health.
D．Identifying the causes of mental health problems.
（4）What does the last paragraph mainly talk about？

B

B

A．Benefits of a regular walk in nature.
B．The focus of the follow-up research
C．An appeal for living in urban regions.
D．Nature's positive effects on women and children.