The two most significant innovations in building design introduced by the romans were the which made construction of the pantheon possible.

The answer to this question is the ARCH and CONCRETE. Arch is a curve structure that was used by the Ancient Romans. Arches have different forms that produce vaults and arcades. While concrete is a composite material that is formed by mixing of cement and by sand, gravel, or stones. 

d.

the northwest territory became the most densely populated area in the u.s.

i dont have that book