Drop manholes at the junctions of sewer lines, are provided if A. Invert level of a branch sewer is more than 60 cm that of the main sewer B. Sewer line runs along a main road C. Ordinary manhole cannot be built D. Two sewer lines intersect

The correct answer is: D. Two sewer lines intersect.

A drop manhole is a type of manhole that is used to connect two sewer lines that are at different elevations. It is typically constructed with a vertical shaft and a horizontal pipe that connects the two sewer lines. The drop manhole allows for the flow of wastewater from the higher elevation sewer line to the lower elevation sewer line.

The other options are not correct because:

• Option A is not always the case. The invert level of a branch sewer can be more than 60 cm that of the main sewer without the need for a drop manhole.
• Option B is not always the case. Ordinary manholes can be built along main roads.
• Option C is not always the case. Ordinary manholes can be built in areas where it is difficult to build a drop manhole.