ANS:E - all the above.

1. Bend: Manholes are often installed at bends or changes in direction in sewer lines. These bends can sometimes cause issues such as blockages or accumulation of debris, so manholes provide access for maintenance and inspection.
2. Junction: Manholes are essential at sewer junctions where multiple sewer lines converge. They allow for easy access to inspect and maintain the connections between different sewer lines.
3. Change of Gradient: Manholes are typically placed at changes in the gradient or slope of the sewer line. Changes in gradient can affect the flow of wastewater, and manholes provide access for inspection and maintenance to ensure proper flow.
4. Change of Sewer Diameter: Manholes are also installed at changes in sewer diameter. When the diameter of the sewer pipe changes, a manhole is often required to accommodate the transition and provide access for inspection and maintenance.