名词
中文翻译：外壳
同义词：
['shell', 'keel', 'core', 'axis']
例句：
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1. The shell of the turtle is a protective covering.
乌龟的壳是一种保护性的覆盖物。
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2. The keel of the ship is the main support structure.
船的龙骨是主要支撑结构。
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3. The core of the planet is its molten interior.
行星的核心是其熔融的内部。
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4. The axis of the planet is its rotational axis.
行星的轴线是其旋转轴。