Inflammation of the periapical tissue is sustained by:
(A) Stagnant tissue fluid (B) Necrotic tissue (C) Microorganisms (D) Pus cells
(C) Apical periodontitis is a chronic inflammatory disorder of periradicular tissues caused by aetiological agents of endodontic origin. 
Persistent apical periodontitis occurs when root canal treatment of apical periodontitis has not adequately eliminated the intraradicular infection (microbes).