CXD砖头书，１７０页１３题

According to a 1980 survey, ten percent of all United States citizens over the age of sixteen are functionally illiterate.
Therefore, if the projection that there will be 250 million United States citizens over sixteen in the year 2000 is correct,
we project that 25 million of these citizens will be functionally illiterate.

Which of the following, if true, would most weaken the conclusion drawn by the author of the passage above?

  A. The percentage of high school graduates who do not go on to college has grown steadily over the past two decades.
  B. From 1975 to 1980 there was a three- percent decrease in the rate of functional illiteracy among the United States citizens over the age of sixteen.
  C. Many United States citizens included in the 1980 survey would be included in a survey conducted in the year 2000.
  D. Surveys that are improperly designed usually provide inaccurate results.
  E. In 1980 sixty-five percent of all United States citizens were over the age of sixteen.
答案说选ｂ，我认为不对．因为1975到1980有下降率，不代表1980到2000年也有下降率．也不代表1980到2000年没有下降率．说明不了问题．其它四个选项也没有说明1980年到2000年之间人口结构的变化．请ＮＮ审阅
题目问的是most weaken ，相比较而言B是最好的选项。当然，楼主的思路很有道理。但是某位牛人曾经说过GMAT不是追求真理，选一个最合适的答案就成了。

答案说就推理而言，其隐含假设是16岁的美国公民文盲比例不变，因此反对就应变化点，而B刚好指出了变化点。如果descrease变为increase就好了
不论是decrease还是increase，只要文盲率有变化，就weaken了题目的观点。

我选了D，设计不当的调查通常提供不准确的结果
我觉得D好像有点牵强了。