I heard some cackling in the background ( I hope they were n't watching it on the monitor ) . I tried to do it as quickly as possible . A couple of times , someone tried to open the door .

Select from: (I) There is a wild animal inside making this noise. (II) Someone is choking , making a cackling sound. (III) None of the above choices .. (IV) Someone is watching them and laughing.
Q: What is a likely source of the cackling that was heard by the speaker ?
(IV)