看了以前的讨论，还是对划线部分的解释不理解，请大虾帮忙

104. Spending on research and development by LACE w:st="on">United StatesLACE> businesses for 1984 showed an increase of about 8 percent over the 1983 level. This increase actually continued a downward trend evident since 1981 – when outlays for research and development increased 16.4 percent over 1980 spending. Clearly, the 25 percent tax credit enacted by Congress in 1981, which was intended to promote spending on research and development, did little or nothing to stimulate such spending.

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

(B) Business spending for research and development in 1985 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 increased more than it did.

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

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

104.

The conclusion that the tax credit did nothing to stimulate spending on research and development would not be true if, without the credit, such spending would have been even lower than it actually was. Thus choice D must be true for the conclusion to be true and is the best answer.

Since a tax credit generally improves business profits, if the conclusion is true choice A is unlikely to be true. If the tax credit was ineffective, some other factors must determine the level of spending, and could lead to much higher levels of spending in 1985 (against choice B), and could render a higher level of tax credit ineffective (against choice C), but it could be that credits are generally effective (against choice E).

我不是大虾，让我试试！请指正

但正是那个退税起了作用。

我觉得这句就是说即使退税rarely effective,也许在这个case上它恰好就effective 呢？所以E是没有道理的。