They have found a way to make waste water clean.如何让改为被动语态

They have found a way to make waste water clean.如何让改为被动语态

A way to make waste water clean have been found by them.

A way to make waste water clean was found by them.

waste water is cleaned by their way. 这句话 已经很好了 还需要 改成什么 被动语态做什么...

A way to make waste water clean have been found by them. 或者
A way have been found by them to make waste water clean .
一楼的不对，时态搞错了。