How is drainage invert elevation determined in pipe drainage design?

How is drainage invert elevation determined in pipe drainage design?

• A. It is the highest interior pipe elevation used to ensure proper slope and flow; derived from design elevations, slope, and pipe diameter.
• B. It is the lowest interior pipe elevation used to ensure proper slope and flow; derived from design elevations, slope, and pipe diameter.
• C. It is the top of the trench
• D. It is the surface grade

Answer: (B)