Перепишите предложения и переведите их, обращая внимание на разные значения слов

Перепишите предложения и переведите их, обращая внимание на разные значения слов it, that, one.

1.People are beginning to realize that environmental problems are their own ones.
2.That ecological problems have become the most important ones at present is evident.
3.It is difficult for mankind to predict changes in the environment.
4.Lead is a toxic substance that can be quickly absorbed by the human organism.