A. Nature. B. Better treatment. C. Experienced doctors. D. Good medicine。

　　15. According to the passage, which of the following statements is TRUE?

　　A. Different aspects(方面) of an urban environment, such as the crowded streets, can lead to an increase in self-control。

　　B. Small changes in urban design cannot reduce the negative side effects of city life。

　　C. For the first time in history, the earth’s population is more urban than rural。

　　D. A walk down a busy city street will improve brain performance。