回答主旨題，一種方法是先回顧文本大意、每段大概內容或者文章中反復重申的主題。然后選出相對應的選項。或者采用排除法，剔除錯誤選項。過程中注意兩點：

  一是要基于文本，而不是自己想當然。所以可以排除文章未提到的內容或者根據文章進行衍生的內容。例如：一篇表達科技是好東西的文章并不一定意味著作者認同科技是解決問題最好的方法。一定要警惕某些選項中的觀點用力過猛，不符合文章本意。

  二是該選項表達的內容至少適用于概括文章的大部分。也就意味著，做主旨題，不能將注意力放在文章的細節上，而是大意上(big picture)。有些選項呈現的只是文章的細節內容，不足以成為文本大意。故要選擇在整個文本中深入討論或在各段多次重申的選項內容。

  主旨題的出題形式大致如下：

  The passage primarily focuses on?

  The passage is primarily concerned with?

  The main purpose of the passage is to?

  等等。

  請看一例：

  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."

  題目：

  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：主旨題的解答也涉及到對文章大意的理解。我們先看下文章的大意脈絡。

  第一段引出新的發現：一種tiger moth可以通過干擾蝙蝠的聲納定位系統避免被捕食。

  第二段展示傳統的觀點：被捕食者發出干擾信號，讓捕食者誤以為它們有毒，從而避免被捕食。

  第三段展示實驗過程，驗證新的發現。

  第四段反駁傳統觀點。

  第五段展示tiger moth干擾蝙蝠聲納定位系統的表現形式。

  第六段猜想。

  故根據對文章大意脈絡的梳理，我們可以知道選項B正確。選項A強調蝙蝠捕捉飛蛾的方式，文章探討的是tiger moth如何避免被捕食。選項C不符，選項C概括了第五段的內容，不足以概括全篇。選項D強調原因。文本強調的不是原因，而是機理。

  選項B正確。

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