This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Environment, Subject name: Social & Preventive Medicine
If the second cup of Horrock's apparatus show blue color after 3 drops of starch-iodine indicator, what dose of bleaching power is required to disinfect 910 L? 
Select and explain. 
A. 8gm, B. 30gm, C. 4gm, D. 15gm
A: 8gm
Answer: A: 8gm 
Explanation: Dose = n x 2 gm for 455 litres n = no, of first cup which show distinct blue colour * n=2 (2nd cup - color change) n x 2 for 455 litres 4 gm for 455 litres 455 litres - 4 gm