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  回答主旨题，一种方法是先回顾文本大意、每段大概内容或者文章中反复重申的主题。然后选出相对应的选项。或者采用排除法，剔除错误选项。过程中注意两点：

  一是要基于文本，而不是自己想当然。所以可以排除文章未提到的内容或者根据文章进行衍生的内容。例如：一篇表达科技是好东西的文章并不一定意味着作者认同科技是解决问题最好的方法。一定要警惕某些选项中的观点用力过猛，不符合文章本意。

  二是该选项表达的内容至少适用于概括文章的大部分。也就意味着，做主旨题，不能将注意力放在文章的细节上，而是大意上(big picture)。有些选项呈现的只是文章的细节内容，不足以成为文本大意。故要选择在整个文本中深入讨论或在各段多次重申的选项内容。

  主旨题的出题形式大致如下：

  The passage primarily focuses on?

  The passage is primarily concerned with?

  The main purpose of the passage is to?

  等等。

  请看一例：

  This passage is adapted from Lynne Peeples “Moths Use Sonar-Jamming Defense to Fend Off Hunting Bats.” © 2009 by Scientific American

  An insect with paper-thin wings may carry much the same defense technology as some of the military's heavy-duty warships. The finding that a species of tiger moth can jam the sonar of echolocating bats to avoid being eaten seems to be the "first conclusive evidence of sonar jamming in nature," says Aaron Corcoran, a biology PhD student at Wake Forest University and the lead author of the paper reporting the discovery. "It demonstrates a new level of escalation in the bat–moth evolutionary arms race."

  Before Corcoran's study, scientists were puzzled by why certain species of tiger moths made sound. Some speculated that the moths use it to startle bats. A few pointed to its potential interference with their echolocation. General consensus, however, fell with a third hypothesis: clicks function to warn a predator not to eat the clicking prey because it is toxic, or at least pretending to be.

  To test these hypotheses, Corcoran and his team pitted the tiger moth Bertholdia trigona against the big brown bat Eptesicus fuscus, a battle frequently fought after sundown from Central America to Colorado. High-speed infrared cameras and an ultrasonic microphone recorded the action over nine consecutive nights. The process of elimination began. If moth clicks served to startle, previous studies suggested the bats should become tolerant of the sound within two or three days. "But that's not what we found," says Corcoran, explaining the lack of success bats had in capturing their clicking prey even through the last nights of the study.

  How about the toxic warning theory? If this were the case, according to Corcoran, bats would not find the moths palatable or, if they were indeed tasty, they would quickly learn they'd been tricked. Either way, bats should start to ignore the moth's unique ultrasonic clicks. Also, bats partook readily when offered B. trigona that lacked the ability to click, and they kept coming back for more. This attraction also held true for clicking B. trigona: The predators persisted after their prey despite only reaching them about 20 percent of the time. Bats actually launched four times as many successful attacks against a control group of silent moths. These findings are "only consistent with the jamming hypothesis," Corcoran notes. "But the most distinctive evidence was in the echolocation sequences of the bats."

  Normally, a bat attack starts with relatively intermittent sounds. They then increase in frequency—up to 200 cries per second—as the bat gets closer to the moth "so it knows where the moth is at that critical moment," Corcoran explains. But his research showed that just as bats were increasing their click frequency, moths "turn on sound production full blast," clicking at a rate of up to 4,500 times a second. This furious clicking by the moths reversed the bats' pattern—the frequency of bat sonar decreased, rather than increased, as it approached its prey, suggesting that it lost its target.

  The biological mechanism behind the moth's defense strategy is still unclear to researchers. "Most likely, moth clicks are disrupting the bat's neural processing of when echoes return," Corcoran says. Bats judge how far away a moth is based on the time delay between making the cry and its audible return. This "blurring" of the bat's vision, he explains, "may be just enough to keep the moth safe."

  题目：

  The passage is primarily concerned with

  (A) the ways Eptesicus fuscus bats capture moths.

  (B) the discovery that tiger moths can jam bats’ sonar.

  (C) how the tiger moths’ clicking defense works.

  (D) why tiger moths developed defenses against bats.

  解析：主旨题的解答也涉及到对文章大意的理解。我们先看下文章的大意脉络。

  第一段引出新的发现：一种tiger moth可以通过干扰蝙蝠的声纳定位系统避免被捕食。

  第二段展示传统的观点：被捕食者发出干扰信号，让捕食者误以为它们有毒，从而避免被捕食。

  第三段展示实验过程，验证新的发现。

  第四段反驳传统观点。

  第五段展示tiger moth干扰蝙蝠声纳定位系统的表现形式。

  第六段猜想。

  故根据对文章大意脉络的梳理，我们可以知道选项B正确。选项A强调蝙蝠捕捉飞蛾的方式，文章探讨的是tiger moth如何避免被捕食。选项C不符，选项C概括了第五段的内容，不足以概括全篇。选项D强调原因。文本强调的不是原因，而是机理。

  选项B正确。

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