Sentence Correction

The skill and the precision of the Anasazi, ancient
inhabitants of the Southwest, in measuring the
movements of the Sun and Moon is evidenced not
only at Chaco Canyon but at a number of other sites.
(A) in measuring the movements of the Sun and
Moon is evidenced not only at
(B) in measuring the movements of the Sun and
Moon are evidenced not only at
(C) in measuring the movements of the Sun and
Moon is evidenced at not only
(D) to measure the movements of the Sun and Moon
is evidenced at not only
(E) to measure the movements of the Sun and Moon
are evidenced not only at

OA is B

why is e incorrect?