Here are the completed sentences with corrections:

1. The house hasn’t been repaired since it **was** built.
2. Glass is usually made of **silica** (or **glass** is usually **made from** sand).
3. The statue was cut out of stone **(correct as is, if you want to keep)**.
4. She asked the guide where Phong Nha cave was the **most beautiful** cave in Vietnam **(correct as is, if you want to keep, or change to "one of the most beautiful")**.
5. When Andrew saw the question, he **knew** the answer immediately.

### Mistakes and Corrections
| Mistakes | Correction |
|------------------------------|----------------------------------------|
| since it built | since it **was** built |
| made of sand | made **from** sand |
| cut out of stone | **(if needed, keep as is)** |
| most beautiful cave | **(if needed, keep as is or change)** |
| was knowing | **knew** |