"A" is the main soil stack that leads to the main sewer pipe. What is "B"?
a. The riser for a passive under-slab radon mitigation system.
b. Discharge pipe for a permanently-installed sump pump.
c. A second, separate main soil stack serving a 2nd floor condominium.
d. An interior (aesthetic) downspout connecting flat roof scuppers to the city storm drain system.