The following are multiple choice questions (with answers) about medical knowledge.   **Question:** 4th generation apex locators are based on?   (A) Frequency based   (B) Impedence based   (C) Multiple frequency ratio based   (D) Resistance based
['(C) Multiple frequency ratio based <> answer', 'The following are multiple choice questions (with answers) about medical knowledge. <> instruction', '(A) Frequency based \n (B) Impedence based \n (C) Multiple frequency ratio based \n (D) Resistance based <> possible answers', '  4th generation apex locators are based on? <> context@']