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If the wind becomes gusty after a period of calm, you should seek shelter. The sky needs careful watching too. Gradual darkening and “boiling” clouds should quicken your pace. Lightening and thunder are common enough storm indicators, but few people realize that the brightness of the lightening is not nearly as important as the number of lightning flashes. The signs of an oncoming storm are many, and a person’s life may depend on his ability to interpret them,
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主题句也可能在段落当中:
From one perspective, this huge customer base represents a hair-raising social liability. Surgical and hospital costs fro regular heart transplants run as high as $500,000. these procedures haven’t burdened the medical system so far – but only because the supply of transplantable hearts has been limited to about 2000 a year. Abiomed plans to price its heart between $75000 and $10000 initially, and with volume production, the price could fall as low as $10000. However, even at the lower price, artificial hearts are an issue that will lead into moral quicksands, says medical ethicist David Steinberg of the Lahey Clinic in Burlington, Mass. What happens, Steinberg muses, “if heart replacement – and intervention directly and visibly linked to who will live and who will die—becomes available only to those who can afford it?”
2 标志词：有些词汇对于我们正确判断一段话的主题是至关重要的。在雅思考试的文章中，主要有两类：强转折性的词、表示重要性结构的词和表示人物观点结论性的词。
那么这些标志词的具体特征和作用如何呢，且看下回分解。
