Sentence Correction

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
The OG says that in C "them " refers to skills rather than students . But skills is an object of the prepostion "of " and hence , skills cant serve as the subject of the verb . Is it correct ?
I have no doubt that C is incorrect .

Also OG says that "lack " is incorrect beacuse the sentence requires an ongoing action conveyed by the word "Lacking "
I dont understand why this sentence requires an ongoing action for the verb "LACK "