GWD5-39新问题

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

这题答案B结构没有问题,但是句义有疑惑.在2000年发生的事件,用现在进行时解释,会不会有歧义?望牛人指点.