大全 7

7. 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 lo 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

Although I correctly select the answer B.  I still don't  know well what the "So " reprents in answer B?

Thanks

当要指代的是一个动作时，往往用so来作为替代。

这里so指代的是wrote这个动作。