At 10:05, a live gate camera shows Lina boarding a flight in Oslo. At exactly 10:05, a live traffic camera shows the person waving from a city bus in Stockholm. That person ___ Lina.

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Can't be gives the confident negative deduction because simultaneous live observations place Lina and the bus passenger in distant cities. Mustn't be would normally express prohibition, while the evidence here establishes that the two people cannot be identical.