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-B
can anybody explain why A is wrong and what difference does it make if use "it".

hi -

"it" is a pronoun. pronouns must stand for NOUNS that are IN the sentence.

there is no NOUN in the sentence that stands for the idea of writing about major figures in black history (that's what the "it" is trying to refer to).

Thanks RON
one more thing, Does C converys something else?

you can't use "that" as a standalone pronoun.
"that" is only acceptable if it's part of a parallel structure, such as a comparison.
for instance:

the statue located in the great hall is much larger than that located in the foyer.

the population of the united states is much larger than that of canada.

Thanks Ron
Learnt a lot from your explanations

can you pls explain option E also

how to kill option E?
thanks in advance

_________________
Many of the great achievements of the world were accomplished by tired and discouraged men who kept on working.

(e) is subtle.

it's essential to realize that there are two kinds of "it":
(1) it is ADJ that/to... --> this is the only instance in which "it" is allowed to function WITHOUT a noun as antecedent.
(2) "it" as a normal pronoun.

examples of (1):
(*) it is obvious that we are going to be late.
(**) it is easy to hit the ball. --> notice that "the ball", NOT "it", is the object of "hit" in this case!

now, note:
(***) the ball is easy to hit. --> notice that "the ball" is the object of "hit" again.
(****) it is easy to hit. --> here "it" stands for the ball. this sentence would NOT be ok on its own; it would have to be preceded by some other sentence indicating that "it" = "the ball".

the correct answer here is structured like (**).
the incorrect choice (e) is structured like (****). since "do" can't be used alone (it MUST have an object), the "it" is forced into the role of being that object. the problem, of course, is that "it" is thus a pronoun - which doesn't stand for anything.

in the correct answer, do so doesn't require an object, so "it" is free to behave as it does in (**) above.