2. 外语
  3. The pearl is a lustrous rounded gem formed inside a mollusk (软体动 物) . Since pearls may be formed by any shelled mollusk, the mos

The pearl is a lustrous rounded gem formed inside a mollusk (软体动 物) . Since pearls may be formed by any shelled mollusk, the mos

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问题     The pearl is a lustrous rounded gem formed inside a mollusk (软体动
物) . Since pearls may be formed by any shelled mollusk, the most valu- 【M1】______
able pearls are obtained from the oysters. Except for imitation pearls, which 【M2】______
are entirely man-made, all pearls are formed from a secretion (分泌物)
around an irritating body that enters the soft tissue of a mollusk.
    If an irritating body enters the mollusk by accident, a natural pearl 【M3】______
forms. A cultured pearl forms if the irritating body is deliberately placed in
the mollusk by the man. 【M4】______
    Unlike to other gems, pearls have a natural luster, and they do not need 【M5】______
to be polished. Although they are usually white or cream-colored, they may
also be any shade of pink, yellow, green, grey, or black. The most desirable
pearls are white with a creamy or pinkish cast. Black pearls are also very
valuable because of their scarce. 【M6】______
    Imitation pearls are often made to appearing very similar to natural 【M7】______
pearls. Moreover, they do not have the luster of natural or cultured pearls. 【M8】______
Another way of distinguishing between imitation and natural or cultured
pearls is by gently rubbing the pearls through the surface of a tooth. Imita- 【M9】______
tion pearls feel smoothly, while natural or cultured pearls feel rough. 【M10】______
【M1】
选项
答案Since — Although
解析 第一个分句的意思是珍珠可以由任何带壳的软体动物形成，第二个分句的意思是最珍贵的珍珠是由牡蛎形成的。两个分句之间的逻辑关系显然并不是因果关系，而是让步关系，所以应该改成Although。
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