The correct answer is A. 65 to 70%.
Sewage is a mixture of wastewater from toilets, sinks, and other drains, as well as rainwater and groundwater. The percentage of sewage discharge that is assumed for the design of sewers depends on a number of factors, including the climate, the type of sewer system, and the level of development in the area. In general, however, a percentage of 65 to 70% is used as a starting point.
Option B, 70 to 75%, is too high for most areas. Option C, 75 to 80%, is even higher and is only appropriate for areas with very high levels of development. Option D, 85%, is much too high and should not be used for any area.
It is important to note that the percentage of sewage discharge that is assumed for the design of sewers is just a starting point. The actual percentage may need to be adjusted based on the specific conditions in the area.