Transform the active sentences into passive:
1. Gustave Eiffel built the Eiffel Tower for the Paris exhibition of 1889.
2. They used 18,038 pieces of iron to build the tower.
3. The Prince of Wales opened the tower.
4. Over six million people have visited the Eiffel Tower this year.
5. People know the Eiffel Tower all over the world as the symbol of Paris.
6. The workers are painting the Eiffel Tower now.