6. The car is very fast, but it is not very fuel efficient (although)
=>The car

7. The movie was very long, and it was also very boring (as well as)
=>The movie

8. The restaurant has delicious food, but the service is slow (while)
=>The restaurant

9. The book was interesting, but it was also very long (besides)
=>The book

10. The hotel room was spacious, but it was also very expensive (nevertheless)
=>The hotel room