Historians discovered an iron pillar at Mehrauli in Delhi, India. Archaeologists believe the pillar was manufactured during the Gupta period. A notable trait about the pillar is that it has not rusted to this day. What can be concluded from this observation? is it A, B,C, D

A.
During the Gupta period, metalsmiths used a lot of imported metal from neighboring regions.
B.
During the Gupta period, many craftsmen of various trades came to India from foreign lands.
C.
During the Gupta period, metalsmiths practiced advanced techniques of casting iron.
D.
During the Gupta period, craftsmen used Persian techniques to construct pillars of this type.