[求助]请教大全148（sec21-07）

148. 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

指代的问题搞得头都晕了，看到it都怕了。。。

问：C的错误是什么？

c的that指什么？

it应该是形式主语指代不定式短语：to do so