1. A lot of rivers polluted/are polluted. 2. Pollution gathers/gathered in clouds. 3. This factory pollutes/polluted the atmosphere. 4. Acid rain causes/caused by toxic fumes that factories emit. 5. Rubbish can recycle/be recycled. 6. Volunteers collect/are collected rubbish in the parks. 7. Nesting boxes have built/have been built by volunteers. 8. Many species have endangered/have been endangered by pollution recently.

Решение:

1. A lot of rivers are polluted. (Present Simple Passive, т.к. действие произошло в прошлом и имеет результат в настоящем)
2. Pollution gathers in clouds. (Present Simple, т.к. это общее утверждение о процессе)
3. This factory pollutes the atmosphere. (Present Simple, т.к. это факт о действии фабрики)
4. Acid rain is caused by toxic fumes that factories emit. (Present Simple Passive, т.к. кислотный дождь является следствием выбросов)
5. Rubbish can be recycled. (Present Simple Passive, т.к. это возможность, действие совершается над предметом)
6. Volunteers collect rubbish in the parks. (Present Simple, т.к. это регулярное действие волонтеров)
7. Nesting boxes have been built by volunteers. (Present Perfect Passive, т.к. действие завершено, а результат важен сейчас)
8. Many species have been endangered by pollution recently. (Present Perfect Passive, т.к. действие произошло недавно и имеет результат в настоящем)

Ответ: are polluted; gathers; pollutes; is caused; be recycled; collect; have been built; have been endangered.