阅读理解
Could HIV, the virus that causes AIDS, be weakening? The results of a study conducted in Belgium, at the Institute of Tropical Medicine in Antwerp, seem to suggest that in one corner of the world it might be. The report, published in the latest issue of AIDS, a specialist journal, concludes that HIV's ability to replicate (known technically as its virulence) may have decreased since the start of the pandemic. Kevin Arien, the lead author of the paper, stresses that the study is based on a small set of samples and does not prove that HIV's virulence is attenua- ting around the world. However, it does offer new insights into the evolution of the disease. Dr. Arien looked at 24 blood samples collected from untreated patients attending an HIV/AIDS clinic in Antwerp. A dozen of these samples were taken between 1986 and 1989; the other 12 were collected between 2002 and 2003. First, he analyzed the samples to find their viral load (the number of virus particles per cubic centimeter) and the subtype of virus involved. In Europe and North America, the predominant subtype is B; in sub-Saharan Africa, where the epidemic is at its worst, the predominant subtype is C. Most of Dr. Arien's samples were of subtype B. Having done this analysis, he paired the samples off for a series of replicative 'duels'. Each sample from the earlier series was matched with the most similar one from the later series, and they were placed in identical cell cultures to see which would multiply the most. The result was that 75 % of the viruses from 2002-2003 were less virulent than apparently similar counterparts from 1986-1989—a statistically significant observation. Dr. Arien's caution is sensible, at least until someone replicates the work elsewhere. But his conclusion is not necessarily surprising. Such viral attenuation, as it is known, is one way that vaccines are produced. What causes attenuation in wild viruses, though, is a matter of speculation. Dr. Arien believes that in this case the attenuation could be the result of what he calls 'serial genetic bottlenecks' during transmission from host to host. These act to reduce the genetic diversity (and thus the replicative fitness) of the virus. Genetic diversity is known to be an important component of HIV's virulence. But what might cause the bottlenecks is still unclear. A second reason for caution besides the small size of the study is, as Geoffrey Garnett, a professor of microparasite epidemiology at Imperial College, London, points out, that the ability of a virus to infect cells in a test-tube is not the same as its ability to cause disease and death in a human host. Nevertheless, Dr. Arien's result is intriguing, and surely worth following up in a larger piece of research.
单选题
The word 'attenuating' (Line 5, Para. 1) most probably means ______.
【答案解析】推理判断题。第二段第二句明确指出十二个血样采自1986到1989年间，而选项A增加了无关信息(died)，可排除；从研究过程和结果来看，Dr. Arien研究的目的并不是艾滋病的成因，而是它的变化情况，排除B；本段中指出Europe and North America和sub-Saharan Africa两个地方的predominant subtype不一样，所以选项C正确；选项D与文意相反。
单选题
Which of the following is true according to the text?
【正确答案】
D
【答案解析】推理判断题。文中的血样采自Antwerp的诊所，其结果不能证明欧洲和非洲的情况，选项A误解了第二段的例子，故错误；由第二段中的they were placed in identical cell cultures可以判断出选项B错误；第四段第一句明确说明导致病毒弱化的原因还在调查中，所以C错误；选项是第二段最后一句的同义转述。
单选题
By saying 'Dr. Arien's caution is sensible' (Line t, Para. 3), the author implies that ______.
【正确答案】
A
【答案解析】语义理解题。本题关键是理解caution一词，它指代的是第一段倒数第二句Arien博士强调：the study...does not prove that...is attenuating一句，而且后面的at least…说明研究还应该继续，即A。B项说，Arien博士的研究是不正确的，文中一直在对他的研究结果作肯定，此项与文章意思相悖；C项文中未提及；D最具混淆性，该项表达的意思是Arien's caution的原因，而非作者所要imply(暗示)的内容。