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Complete the archaeologist's notes about a bizarre and highly unscientific discovery in an ancient Egyptian tomb.
We found a modern digital watch resting next to the mummy. The ancient Egyptians _________________________ invented this technology! Professor Jones was the only person in the tomb yesterday, so he _________________________ dropped it while taking photographs.
The correct answers are couldn't have and must have.
Couldn't have is used for a strong negative deduction in the past—it is historically impossible for ancient Egyptians to have invented digital watches.
Must have expresses a strong positive deduction—since Professor Jones was the only person there, it is the only logical conclusion that he dropped it.