Rewrite the sentences (1-8) in the passive. In which sentences can we omit the agent Why

2. The ancient forest was destroyed by fire.
3. The lake has been visited by millions of people.
4. The town was rebuilt (by them) after the huge flood.
5. A book about the wildlife of Kazakhstan was written by Alexander N.Formosow.
6. A documentary film about deforestation was being shown last week (by the local cinema).
7. The first Earthship was created by Mike Reynolds.
8. The plants are being watered (by him) at the moment.