SC-Sociological Study

Everyone participating in the early sociological study committed a crucial methodological error by failing to fully consider alternative ways of classifying their data
A) by failing to fully consider alternative ways of classifying their
B) by failing fully to consider alternative ways of classifying their
C) they failed to consider alternative ways to classify their
D) by not fully considering alternative ways to classify the
E) by not considering fully alternative ways of classifying his or her

.
.
.
.

D is best
A B and C are out because "everyone" is singular and needs a singular referrent
E is out for misplacing adverb after verb "considering"

I still contend that this is perhaps not the correct explanation. Lets see what Pret has to say :)