OG13-86

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) a phenomenon that is explained not just by the fact that drugs are becoming more expensive but also by the fact that doctors are writing
(C) a phenomenon occurring not just because of drugs that are becoming more expensive but because of doctors having also written
(D) which occurred not just because drugs are becoming more expensive but doctors are also writing
(E) which occurred not just because of more expensive drugs but because doctors have also written
答案是B

请教下C中 not just because of drugs that .... but because of doctors .... 不是平行的吗？"that are becoming more expensive"定语从句修饰"drugs"; "having also written many more prescriptions for higher-cost drugs " 动宾形式修饰"doctors"。是不是由于"having also written"的时态错应该用"writing"啊？！

但OG解释说是：The phrasing drugs that are becoming and doctors having also written are awkward and confusing; the placement of also is incorrect. 那么awkward and confusing在哪里呢？

请NN来回答下吧！！