Correct option is D
Circular sewers are suitable for separate sewerage system. This sewer is easy to construct. The inner surface of this sewer is smooth so the discharge is uniform not variable and no chances of settling of suspended particle.
When the storm water drainage is taken along with sewage it is called a combined system.
Oval shape sewer is also known as egg shaped sewer. These sewers are preferred for combined sewerage system where the dry weather flow is small as compared with the total capacity of the sewer. It is carrying varying discharge.
