GMAT Sentence Correction (SC) | Re: Q) The higher productivity rate of some Japanese companies

Q) The higher productivity rate of some Japanese companies relative to comparable US companies is due not totheir faster production lines, which are in fact slower in the Japanese companies, but to a greater amount of engineers and equipment per production worker.

(A) their faster production lines, which are in fact slower in the Japanese companies, but to a greater amount of engineers and
(B) their faster production lines, since they are in fact slower in the Japanese companies, but to a greater