Here are the completed sentences:

1. The **cause** of the fire at the hotel last night is still unknown.
2. The **advantage** of living alone is that you can do what you like.
3. Money isn’t the **solution** to every problem.
4. There has been an **increase** in the number of road accidents recently.
5. There are some **differences** between British and American English.
6. When Paul left home, his **attitude** seemed to change.
7. The firm closed down because there wasn’t enough **demand** for its product.
8. When I opened the envelope, I was delighted to find a **cheque** for £500.
9. Bill and I used to be good friends but I don’t have much **contact** with him now.
10. Carol has decided to give up her job. I don’t know her **reason** for doing this.
11. I’ve had an **invitation** to Laura’s barbecue.
12. The accident caused some **damage** to the car.
13. Unfortunately there was some **difficulty** with the arrangement.
14. We need to provide more shelters for the homeless, so there’s a **demand** for more houses.
15. The two companies are completely independent. There is no **connection** between them.