SC - unusual

[Xbond]

Hi there,

I don't understand how to attack this SC how to resolve
What is the concept behind; A clear explanation is requested

At a time when it was unusual to do it, Dorothy Sterling wrote about such major figures of Black history as Harriet Tubman and W. E. B. Du Bois.

(A) it was unusual to do it
(B) it was unusual to do so
(C) doing that was unusual
(D) that was an unusual thing to be doing
(E) it was not usual to do

OA is B

[eveaanr]

(A) it was unusual to do IT : Here it refer to what? Noun has not been mentioned yet.
(B) it was unusual to do so : Correct choice
(C) doing that was unusual : Here that refers to what? Noun has not been mentioned yet.
(D) that was an unusual thing to be doing : Wrong construction-thing to be doing
(E) it was not usual to do : Feels incomplete.Further, prefer to use 'unusual' rather than 'not usual'.