求助

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Spending on research and development by UnitedStates businesses for 1984 showed an increase of about 8 percent over the 1983level. This increase actually continued a downward trend evident since 1981 –when outlays for research and development increased 16.4 percent over 1980spending. Clearly, the 25percent tax credit enacted by Congress in 1981, which was intended to promotespending on research and development, did little or nothing to stimulate suchspending.

The conclusion of the argument above cannot be trueunless which of the following is true?

(A) Business spending on research and development isusually directly proportional to business profits.

(B) Business spending for research and development in1985 could not increase by more than 8.3%.

(C) Had the 1981 tax credit been set higher than 25%,business spending for research and development after 1981 would have increasedmore than it did.

(D) In the absence of the 25% tax credit, businessspending for research and development after 1981 would not have beensubstantially lower than it was. D

(E) Tax credits market for specific investments arerarely effective in inducing businesses to make those investments.

请问这道题C为什么不对啊

mnimi

题干说tax credit 1981年 有25%, 1980-1981年 研究经费增长了16.4%.
C选项，如果1981年tax credit > 25%, 1980-1981年研究经费增长> 16.4%
题目问如何证明25%和+16.4%无关。
C无法证明它们的关系。因为还有可能有tax credit以外的其他原因对研究经费增长产生影响，万一tax credit增长时，真正的原因也增长了，所以研究经费增长了呢?
相反，D则能很好证明，因为如果没有tax credit, 仍然有研究经费增长，就可以证明此原因和tax credit无关。