This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Nutrition and health, Subject name: Social & Preventive Medicine
Double edged sword is - 
Select and explain. 
A. Chlorine, B. Fluorine, C. Lead, D. Selenium
B: Fluorine
Answer: B: Fluorine 
Explanation: Fluorine is often called double edged sword. Prolonged ingestion can lead to dental and Skeletal flurosis.REF. PARK'S TEXTBOOK OF PREVENTIVE AND SOCIAL MEDICAL 21ST EDITION. PAGE NO - 577