That creates a problem in C, which says "...lack of skills is so large as to be difficult to absorb..." The implication is that the lack of skills is "difficult to absorb". That's not the intended meaning; the intended meaning is that it's difficult to absorb the students. (Also, the following "them" is incompatible with this construction.)
C also starts with "students ... lack of math skills", which is incorrect; it should be students' lack of skills. (This is like trying to say "Ron car" instead of "Ron's car".) With the modifier after "students" it's pretty much impossible to write this correctly, so the correct answers don't even try"”they write the sentence with "lack" as a verb instead.
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