Because the financial review covered only the last few fiscal years, and therefore the investigators were unable to determine the extent to possible earlier overpayments.
(A) and therefore the investigators were unable to determine the extent to possible
(B) so therefore the investigators were not capable of determining the possible extent of
(C) therefore the investigators were unable to determine the possible extent of
(D) the investigators were not capable of determining the possible extent of
(E) the investigators were unable to determine the extent of possible

why the choice is not D?