This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Environment and Health, Subject name: Social & Preventive Medicine
Ortho-toluidine test measures the levels of: 
Select and explain. 
A. Combined chlorine, B. Both free and combined chlorine, C. Nitrates in water, D. Hardness of water
B: Both free and combined chlorine
Answer: B: Both free and combined chlorine 
Explanation: Ans. b. Both free and combined chlorineTest for chlorination of waterOrtho-toluidine test (OT)Ortho-toluidine Arsenite test (OTA)* Free (residual) chlorine* Free chlorine* Free and Combined chlorine* Combined chlorineOTA test is better than OT test as:Detects both free and combine water chlorine separately.Not affected by interfering substances (Nitrites, Iron, and Manganese).