Why have been is not required.

A surge in new home sales and a drop in weekly
unemployment claims suggest that the economy might
not be as weak as some analysts previously thought.

(A) claims suggest that the economy might not be
as weak as some analysts previously thought

(B) claims suggests that the economy might not
be so weak as some analysts have previously
thought

(C) claims suggest that the economy might not be
as weak as have been previously thought by
some analysts

(D) claims, suggesting about the economy that it
might not be so weak as previously thought by
some analysts

(E) claims, suggesting the economy might not be as
weak as previously thought to be by some
analysts


Here, I can eassily eliminate B,D,E, because the subject is plural and simple only "suggest" is required.

My doubt is why HAVE BEEN not required in option C.

Please advise me and correct me if am wrong...