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Complete the roommates' conversation about their very weird cat by choosing the correct modal verbs of deduction.
Roommate A: "Why is Whiskers staring at the blank wall?"
Roommate B: "I don't know. There _________________________ be a tiny bug crawling there that we can't see."
Roommate A: "No way, the wall is perfectly clean. She _________________________ be listening to mice in the pipes!"
Roommate B: "Well, she's purring happily and rubbing her face on the floor, so she _________________________ be scared of whatever it is."
could
Could expresses a possibility. Roommate B is guessing about a bug but isn't sure.
must
Must shows strong certainty based on evidence. Since the wall is clean, Roommate A concludes that hearing mice is the only logical explanation.
can't
Can't shows strong certainty that something is false. Because the cat is purring happily, it is impossible that she is scared.