ANS:D - none of these.

1. Top of smaller sewer is kept lower: This arrangement is not typically used because it would result in sewage flowing uphill from the larger sewer into the smaller one, which goes against the natural flow of gravity.
2. Top of larger sewer is kept lower: Similarly, this arrangement is also not commonly used because it would require sewage to flow uphill from the smaller sewer into the larger one, which is not practical.
3. Tops of both the sewers are at the same level: While it's theoretically possible to have both sewer tops at the same level, this arrangement could lead to potential flow issues and backups, especially if one sewer has a higher flow rate than the other.

Now, let's discuss why "none of these" might be the correct answer: In practice, sewer junctions are designed with careful consideration of hydraulic principles and gravity flow. The aim is to ensure that sewage flows smoothly from one sewer to another without encountering obstacles or creating backups. Depending on factors such as flow rates, pipe sizes, and terrain, the design of sewer junctions may vary. In some cases, engineers may use specific design features such as drop manholes or invert adjustments to control the flow direction and elevation of sewage at junctions. These design elements help to maintain the proper flow gradient and prevent issues like surcharging or blockages. Therefore, the most appropriate design for a sewer junction depends on various factors, and it's essential to evaluate each situation individually to determine the optimal configuration. As such, "none of these" could be the correct answer if none of the provided options accurately represent common design practices for sewer junctions.