题目选项分析

In 2000, a mere two dozen products accounted for half the increase in spending on prescription drugs, 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 many more prescriptions for higher-cost drugs.

In 2000, a mere two dozen products accounted for half the increase in spending on prescription drugs, which occurred not just because of more expensive drugs but because doctors have also written many more prescriptions for higher-cost drugs.