Glass Curtain Walls in Which Aluminum Framing Sections Are Specially

Glass curtain walls in which aluminum framing sections are specially profiled for a particular project are

A) rare because of the prohibitive cost of manufacturing custom profiles.
B) uncommon because of the extremely high cost of manufacturing custom profiles.
C) fairly common because the cost of custom profiles can be recovered from one large project.
D) not uncommon because the cost of custom profiles can be recovered from a few repeat mid- sized to large projects.