用单数或复数都可以:若把 a set of keys 视为一个整体,则用单数谓语动词;若侧重考虑 a set of keys 中的每一个 key,则用复数谓语动词。如:
A set of keys was swinging from his belt. 一串钥匙在他的腰带上摆来摆去。
A set of keys were swinging from his belt. 一串钥匙在他的腰带上摆来摆去。
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