A 70 year old man is diagnosed to have Legionnaire's disease after he returns home attending a convention. He could have acquired it:

Legionnaires' disease is the designation for pneumonia caused by Legionella. Inhalation of the aerosol in the air-conditioned room is a proposed mode of acquiring this infection. However according to Harrison "Cooling towers and evaporative condensers have been overestimated as sources of Legionella. In many outbreaks attributed to cooling towers, cases of Legionnaires' disease continued to occur despite disinfection of the cooling towers; drinking water was the actual source". So among the given options C is the most appropriate answer. Ref: Harrisons principles of internal medicine, 18th edition, chapter 147