名词
中文翻译：Unknown
同义词：
['pebble', 'gravel', 'cobblestone']
例句：
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1. The riverbed is covered in smooth, round pebbles.
河床覆盖着光滑的圆石。
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2. The old house was built on a bed of gravel.
这座旧房子建在砾石上。
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3. The cobblestone streets are well-traveled and worn down by years of foot traffic.
鹅卵石街道经过多年的行人磨损，非常著名。