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 occurring not just because of drugs that are becoming more expensive but because of doctors having also written many more prescriptions for higher-cost drugs.

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.