【正确答案】 C

【答案解析】参看原文Recently an association has been found between the levels of particles in the air and death rates in North American cities．The reason for this association is not understood and as vet there is no evidence this occurs in Australia．意思是说，最近发现北美空气中微粒的浓度与死亡率之间存在联系，但是原因尚不知晓，而且在澳大利亚并未发现此类现象。