本题对应原文 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，即 aragonite 不稳定，会变成更稳定的形式，所以答案是 C 选项。A 选项不对，aragonite 实际上会发生改变，并不是保持不变；B 选项的 soft tissue 和 hard shells 的比较原文未提到；D 选项的 fossil shells 和 fossil skeletons 的比较原文也未提到。
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