In the traditional Japanese household, most clothing could be packed flatly, and so it was not necessary to have elaborate closet facilities.

A. flatly, and so it was not necessary to have elaborate closet facilities 29%
B. flat, and so elaborate closet facilities were unnecessary 29%
C. flatly, and so there was no necessity for elaborate closet facilities 29%
D. flat, there being no necessity for elaborate closet facilities 14%
E. flatly, as no elaborate closet facilities were necessary

I am between A and B. As I know we must use adverb after to pack. So why B?