a pair of glasses(一副眼镜)做主语时谓语动词用单数 a couple of ，a bottle of 等等这些短语后面如果跟名词的单数或者复数时，谓语v用单数如果是two piars of，那谓语动词一定是复数如：This is a pair of glasses.There are two pairs of glasses.
