The correct answer is: A. Circular sewer
A circular sewer is the most common type of sewer because it is the most efficient shape for carrying wastewater. It has a smooth interior surface that minimizes friction and allows wastewater to flow quickly and easily. Circular sewers are also relatively easy to construct and maintain.
An egg-shaped sewer is a less common type of sewer that is sometimes used in areas with high groundwater levels. The egg shape helps to reduce the amount of infiltration into the sewer system. However, egg-shaped sewers are more difficult to construct and maintain than circular sewers.
A horseshoe-type sewer is a type of sewer that is sometimes used in areas with high volumes of wastewater. The horseshoe shape helps to increase the capacity of the sewer. However, horseshoe-type sewers are more difficult to construct and maintain than circular sewers.
A semi-elliptical sewer is a type of sewer that is sometimes used in areas with high slopes. The semi-elliptical shape helps to reduce the amount of erosion that can occur at the bottom of the sewer. However, semi-elliptical sewers are more difficult to construct and maintain than circular sewers.
In conclusion, the type of sewer which is suitable for both combined and separate system is a circular sewer.