Sentence Correction

It!!!!!
Q.Students in the metropolitan school district lack math skills to such a large degree as to make it difficult to absorb them into a city economy becoming ever more dependent on information-based industries.

(A) lack math skills to such a large degree as to make it difficult to absorb them into a city economy
becoming
(B) lack math skills to a large enough degree that they will be difficult to absorb into a city's economy that
becomes
(C) lack of math skills is so large as to be difficult to absorb them into a city's economy that becomes
(D) are lacking so much in math skills as to be difficult to absorb into a city's economy becoming
(E) are so lacking in math skills that it will be difficult to absorb them into a city economy becoming

ans E
i thought 'it' was used to replace nouns and also it(pronoun) doesn't refer to anything here,does it?

please explain the use of it
thanks in advance