In a residential colony, sewers of diameters 100 mm, 150 mm and 225 mm were laid with a gradient 1 in 120. Which portion of the sewage system does not choke in due course of time? A. 100 mm dia B. 150 mm dia C. 225 mm dia D. All of these

The correct answer is D. All of these.

The gradient of a sewer is the slope of the sewer pipe. The gradient is important because it determines the velocity of the sewage flow. A higher gradient means a faster sewage flow, which helps to prevent clogging.

In the given question, the sewers of diameters 100 mm, 150 mm and 225 mm were laid with a gradient of 1 in 120. This is a good gradient for sewage flow. It is not too steep, which would cause the sewage to flow too fast and could damage the pipes. It is also not too shallow, which would cause the sewage to flow too slowly and could clog the pipes.

Therefore, all of the sewers in the given question are unlikely to choke in due course of time.