In a Stick- Built Glass Curtain Wall, the Rails Are

In a stick- built glass curtain wall, the rails are typically spaced at

A) 4 ft to 6 ft o.c.
B) 2 ft to 4 ft o.c.
C) 6 ft to 10 ft o.c.
D) 10 ft to 15 ft o.c.
E) as needed for the project.