ANS:B - is doubled

If the depth of flow in circular sewers is 0.2 times the full depth, the nominal gradient needs to be doubled. The nominal gradient, also known as the slope or gradient of the sewer, is the ratio of the change in elevation to the length of the sewer. It determines the flow velocity and capacity of the sewer system. When the depth of flow is significantly lower than the full depth, as in this case where it's only 0.2 times the full depth, the sewer may not be operating efficiently. Increasing the slope or gradient of the sewer by doubling it can help improve flow velocity and ensure that the sewer system operates effectively, preventing issues such as sedimentation and blockages.