GWD 05-Q39 [未充分讨论]

Q39:

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.

1. a phenomenon that is explained not just because of more expensive drugs but by the fact that doctors are writing
2. 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
3. a phenomenon occurring not just because of drugs that are becoming more expensive but because of doctors having also written
4. which occurred not just because drugs are becoming more expensive but doctors are also writing
5. which occurred not just because of more expensive drugs but because doctors have also written

Answer is B, but why not C? And any understanding of not just ... but also? I did not find any example for it.

because of doctors having written是不好的表达方法

完成时态的分词如果没记错，gmat是不提倡的。