The following are multiple choice questions (with answers) about medical knowledge.   **Question:** The green stains frequently seen on children's teeth are caused by   (A) Materia alba   (B) Enamel defects   (C) Dentinal defects   (D) Chromogenic bacteria
['(D) Chromogenic bacteria <> answer', 'The following are multiple choice questions (with answers) about medical knowledge. <> instruction', '(A) Materia alba \n (B) Enamel defects \n (C) Dentinal defects \n (D) Chromogenic bacteria <> possible answers', "  The green stains frequently seen on children's teeth are\ncaused by <> context@"]