1. C (could)
2. D (stops)
3. D (solution)
4. D (borrow)
5. B (has appeared)
6. B (in)
7. D (can be prevented)
8. C (is studying in her room)
9. A (one of the biggest)
10. D (could travel)
11. B (in)
12. C (as large as)
13. A (although)
14. B (communication)
15. A (that's very kind of you)
16. C (yes, I share your opinion)