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托福閱讀功能作用題技巧，我們來康康一道人見人怕的功能作用題～
Why does the author mention "aragonite" in the passage?
首先，以題干中的“aragonite”，尋找所在意群的觀點句。
Fossil Preservation Paragraph 2: The term "fossil" often implies petrifaction, literally a transformation into stone. After the death of an organism, the soft tissue is ordinarily consumed by scavengers and bacteria. The empty shell of a snail or clam may be left behind, and if it is sufficiently durable and resistant to dissolution, it may remain basically unchanged for a long period of time. Indeed, unaltered shells of marine invertebrates are known from deposits over 100 million years old. In many marine creatures, however, the skeleton is composed of a mineral variety of calcium carbonate called aragonite. Although aragonite has the same composition as the more familiar mineral known as calcite, it has a different crystal form, is relatively unstable, and in time changes to the more stable calcite.
原文中直接標(biāo)注“aragonite ”位置，該句中以“however”插入形式引出「句間轉(zhuǎn)折」?！妇溟g轉(zhuǎn)折」和「句內(nèi)轉(zhuǎn)折」理解的重點都是找反義詞組，只不過「句內(nèi)轉(zhuǎn)折」反義詞組在一句話內(nèi)成對出現(xiàn)，而「句間轉(zhuǎn)折」是在前后兩個意群中分別出現(xiàn)。
在這段話中，我把however前稱為「轉(zhuǎn)折前意群」，however后稱為「轉(zhuǎn)折后意群」。再次套用轉(zhuǎn)折邏輯的套路，要想做對題一定要理解轉(zhuǎn)折中的反義詞組。在轉(zhuǎn)折前后意群查找中，可以識別出unchanged=unaltered ?VS ?unstable=change。而“aragonite”出現(xiàn)在轉(zhuǎn)折后意群，則其作為栗子強調(diào)”unstable/change“這一關(guān)鍵。
A To emphasize that some fossils remain unaltered for millions of years
B To contrast fossil formation in organisms with soft tissue and in organisms with hard shells
C To explain that some marine organisms must undergo chemical changes in order to fossilize
D To explain why fossil shells are more likely to survive than are fossil skeletons
秒殺時刻到了?。?！四個選項無需精讀，重點看有沒有出現(xiàn)關(guān)鍵詞“unstable/ change“即可，所以馬溜溜確定C選項，必須做對！??！
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