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.
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