the boys是单数还是复数

　　The boys were riding their bikes around the streets.

　　男孩子们骑着自行车在街上兜风。

　　The boys were throwing sticks and stones at the dog.

　　男孩子们朝那条狗扔枝条扔石头。

　　The boys were skipping stones across the pond.

　　那几个男孩子用石块往水塘里打水漂。