The total capacity of a reservoir is 25 million cubic metres and dead storage is 5 million cubic metres. If the average volume of sediment deposition is 0.10 million cubic metre per year, then the usefulness of the reservoir will start reducing after A. 50 years B. 150 years C. 200 years D. 250 years

The correct answer is A. 50 years.

The usefulness of a reservoir is reduced when the sediment deposition exceeds the dead storage. In this case, the dead storage is 5 million cubic metres and the average volume of sediment deposition is 0.10 million cubic metres per year. Therefore, the usefulness of the reservoir will start reducing after 50 years.

Option B is incorrect because the usefulness of the reservoir will start reducing after 50 years, not 150 years.

Option C is incorrect because the usefulness of the reservoir will start reducing after 50 years, not 200 years.

Option D is incorrect because the usefulness of the reservoir will start reducing after 50 years, not 250 years.