According to a model based on NetLogo, which simulates the activities of human being as a clear evidence, it is shown that it is necessary to wear a mask. At the same time, the effect brought by different population density is also deserved to be considered. Under most circumstances, the more people gathering together, the higher the rate of infected is; but a 75people gathering is also acceptable and its rate of infected is even lower than 50-people gathering. Although 75-people seems a good

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... oice, the gathering with less than 50-people gathering will actually be much safer and it is always a good choice to keep a low population density. However, the policies should also consider a reasonable gathering to make sure that the basic facilities, social services and economy system are still working.