本身就是复数的词

　　眼镜glasses; 耳机earphones; 鞋shoes;裤子trousers等词本身是复数。

　　如：My glasses were on the chair just now.

　　但如果表示这双，这副，一双的时候用单数

　　如：There is a pair of chopsticks on the plate. This pair of earphones is for you.