题目

PM2.5 refers to fine particles (2.5 micrometres or smaller in diameter ), which are produced by combustion (氧化），including motor vehicles , power plants , forest fires, and some industrial processes .The WHO tracks air quality ( how much PM 2.5 is in the air ) at 1622 locations in 92 countries -- but all are urban areas , while Pakistan , Egypt and Mongolia are among the most polluted countries according to the report , this only refers to pollution in its cities . Air quality in the Karakoram mountain range or the Gobi Desert will, of course , be fresh .Similarly ,Russia appears to be There’s ______ art room_______ the top floor.    A．an, in         B．a, on         C．an, on       D．a, in