It's dangerous to let pet birds free so it's better to keep them what?
Options:
- slavery
- caught
- caged in
- topfree
- prisoner Hmmm, let me think.

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
user
Companion birds are kept in cages because they are not used to roam in the wild.
So, the answer is caged in.