I will work hard all my life变复数形式They young women are all in white trousers,咋变单数

I will work hard all my life变复数形式
They young women are all in white trousers,咋变单数

We will work hard all our lives

We will work hard all our lives.
不太理解变复数形式的意义。可否详解？

I will work hard all my lives.

the young woman is in white trouser