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1.(к подлежащему) By now the pollution and poisoning of the soil,water and air have reached a critical level.
2.(разделительный) The discharge of dust and gas into the atmosphere returns to the Earth in the form of acid rain
3.Huge areas would be infertile and become uninhabitable.(к сказуемому )
4.(альтернативный) We all have a responsibility to take care of nature .
5.They are obliged to remove factories and plants from cities,use modern technologies,redesign and modify purifying systems( общий)

1) what have reached a critical level by now?2) the discharge of dust and gas into the atmosphere returns to the Earth in the form of acid rain, doesn't it? 3) what would huge areas be doing?4) do we all have responsibility to take care of nature or we should behave ourselves as now?5) are they obliged to remove factories and plants from cities,use modern technologies,redesign and modify purifying systems?