The minimum diameter of a sewer is 15 cm. This is because the sewer must be able to carry the maximum flow of sewage without overflowing. The flow of sewage is calculated based on the number of people living in the area and the amount of water they use. The minimum diameter of a sewer is also based on the type of sewage that will be carried. For example, a sewer that will carry only wastewater will have a smaller diameter than a sewer that will carry both wastewater and storm water.
Option A: 10 cm is too small for a sewer. A sewer with a diameter of 10 cm would not be able to carry the maximum flow of sewage without overflowing.
Option B: 15 cm is the minimum diameter for a sewer. A sewer with a diameter of 15 cm will be able to carry the maximum flow of sewage without overflowing.
Option C: 20 cm is too large for a sewer. A sewer with a diameter of 20 cm would be more expensive to build and maintain than a sewer with a diameter of 15 cm.
Option D: 25 cm is too large for a sewer. A sewer with a diameter of 25 cm would be more expensive to build and maintain than a sewer with a diameter of 15 cm, and it would not be necessary to have a sewer with such a large diameter.