名词
中文翻译：包层
同义词：
['shell', 'layer', 'envelope']
例句：
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1. The outer layer of the onion is very tough.
洋葱的外层非常坚韧。
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2. The metal was wrapped in layers of plastic to protect it.
金属被层层塑料包裹以保护它。
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3. The scientist used a microscope to examine the inner layer of the sample.
科学家使用显微镜检查样本的内部层次。