STANFORD -- Until cultures change radically, the optimum number of people to exist on the planet at any one time lies in the vicinity of 1.5 billion to 2 billion people, about a third of the present number, three California ecologists estimated in an article published in the journal Population and Environment.Jul 11, 1994
疫苗真的安全吗?烈治文山医院华裔专家解答