다시 질문드릴꼐요~

토익 문제인데요.
the plans for the downtown office tower were not designed by us alone, were rather the result of two design firms working together for a final project.

a) or
b) but
c) which
d) both

답이 both라는데 왜 그런거져?

both는 명사역할로 접속사가 없이 the plans were~ , both were 이렇게 나올 수 없지 않나요?
,는 접속사 역할 안된다고 알고 있는데..