单选题 {{B}}第三篇{{/B}}

There are some earth phenomena you can count on, but the magnetic field, some say, is not one of them. It fluctuates in strength, drifts from its axis, and every few 100,000 years undergoes a dramatic polarity reversal —a period when north pole becomes south pole and south pole becomes north pole. But how is the field generated, and why is it so unstable?
Groundbreaking research by two French geophysicists promises to shed some light on the mystery. Using 80 metres of deep sea sediment (沉淀物) core, they have obtained measurements of magnetic-field intensity that span 11 polarity reversals and four million years. The analysis reveals that intensity appears to fluctuate with a clear, well-defined rhythm. Although the strength of the magnetic field varies irregularly during the short tern, there seems to be an inevitable long-term decline preceding each polarity reversal. When the poles flip—a process that takes several hundred thousand years--the magnetic field rapidly regains its strength and the cycle is repeated.
The results have caused a stir among geophysicists. The magnetic field is thought to originate from molten (熔化的) iron in the outer core, 3,000 kilometers beneath the earth's surface. By studying mineral grains found in material ranging from rocks to clay articles, previous researchers have already been able to identify reversals dating back 170 million years, including the most recent switch 730,000 years age. How and why they occur, however, has been widely debated. Several theories link polarity flips to external disasters such as meteor (陨星) impacts. But Peter Olson, a geophysicist at the Johns Hopkins University in Baltimore, says this is unlikely if the French researchers are right. In fact, Olson says intensity that predictably declines from one reversal to the nest contradicts 90 percent of the models currently under study. If the results prove to be valid geophysicists will have a new theory to guide them in their quest to understand the earth's inner physics. It certainly points the direction for future research.

单选题 Which of the following titles is most appropriate to the passage?

单选题 The word "flip" (Pare. 2) most probably means______.

【正确答案】 D

【答案解析】根据文章第二段：法国科学家的分析表明地球磁场强度的变化具有明显的节奏。尽管短期内磁场强度会发生不规则的变化，但每次两极转化之前似乎都不可避免地有一个长期的衰竭期。当两极转化时，地球磁场迅速地获得了能量并开始新一轮的循环。由此，根据上下文的意思可推断，poles flip就是指代前面的polarity reversal。flip指的就是两极的转化，由此可知，D项选择的意思与此项相符。

单选题 What have the two French geophysicists discovered in their research?

单选题 The French geophysicists' study is different from currently prevailing theories in______.

【正确答案】 C

【答案解析】题目询问法国地球物理学家的研究与当今流行的理论的区别。根据文章第三段：当今的理论已能识别地球两极发生转化这一现象，但是地球两极是怎样发生转化和为什么会发生转化，却是一个引起广泛争议的问题，而法国科学家在这—点上提出了自己的观点。C项选择，解释了地球两极的移动，与题意相符。