The correct answer is A. A sewer drops from a height.
A drop manhole is a type of manhole that is used to provide access to a sewer line
that is dropping from a height. It is typically installed at the bottom of a vertical drop in the sewer line, and it allows workers to access the line for inspection, cleaning, or repair.Option B is incorrect because a branch sewer joining the main sewer at a higher level does not require a drop manhole. The branch sewer can simply be connected to the main sewer at the higher level without the need for a drop.
Option C is incorrect because a lamp is not inserted into a drop manhole to check obstructions. A lamp is typically inserted into a manhole to provide light for workers who are inspecting or cleaning the sewer line.
Option D is incorrect because a drop manhole is a type of manhole that is used to provide access to a sewer line that is dropping from a height.