Correct the errors if necessary. 1. Since when are they waiting for us? 2. I have been looking for my passport since I came home. 3. His friends have been having this building for ten years. 4. In recent years, scientists are developing many new machines.

1. Since when have they been waiting for us?
2. I have been looking for my passport since I came home.
3. His friends have had this building for ten years.
4. In recent years, scientists have developed many new machines.