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！