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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.

不是 not only ... but also...

这里， not .... but... 结构。

对于 not only...but also... 的解释:

241. As well as heat and light, the Sun is the source of a continuous stream of atomic particles known as the solar wind.

(A) As well as heat and light, the Sun is the source of a continuous stream

(B) Besides heat and light, also the Sun is the source of a continuous stream

(C) Besides heat and light, the Sun is also the source of a continuous streaming

(D) The Sun is the source not only of heat and light, but also of a continuous stream（D）

(E) The Sun is the source of not only heat and light but, as well, of a continuous streaming

Choice D, the best answer, has no modification errors and uses parallel phrases to complete the idiomatic construction not only... but also. Choices A, B, and C have modification errors: As well as heat and light and Besides heat and light cannot logically modify the Sun, the nearest noun, as grammar requires them to do. This misdirected modification suggests that heat and light are also (in addition to the Sun) a source of the solar wind. Choice B may be faulted for the awkward word order of also the Sun, while C unnecessarily uses streaming rather than the more straightforward stream. Choice E fails to use parallel phrases in the idiomatic construction not only x... but [also] y: of should not appear before not only if it appears after but, and but also is preferable to but. Finally, the use of the gerund streaming rather than the more straightforward noun stream is needlessly awkward.