GMAT Sentence Correction (SC) | In 2000, a mere two dozen products accounted for

Hi, this question has been posted before, but regarding a different point of grammar, hence starting a new thread.

In 2000, a mere two dozen products accounted for half the increase in spending on prescription drugs,a phenomenon that is explained not just because of more expensive drugs but by the fact that doctors are writing many more prescriptions for higher-cost drugs.

(A) a phenomenon that is explained not just because of more expensive drugs but by the fact that doctors are writing

(B)