bread and butter作主语谓语动词用单数还是复数

问题描述

Every day we had to have bread and butter, which was the only food they gave us.
句中的which was the only food they gave us是非限制性定语从句，修饰bread and butter；但bread and butter是两样东西，为什么后面的定语从句谓语用了单数was而没有用复数were呢？请问：bread and butter作主语谓语动词用单数还是复数？谢谢！

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