28. 國際癌症研究機構(IARC)將戶外空氣污染列為何種等級的致癌物？ (A) carcinogenic to humans (Group 1). (B) Probably carcinogenic to humans (Group 2A) (C) Possibly carcinogenic to humans (Group 2B) (D) Not classifiable as to its carcinogenicity to humans (Group 3) (E) Probably not carcinogenic to humans (Group 4)