Modals of Deduction (Certainty)

Which modal expresses an impossible/negative deduction: «mustn't» or «can't»?

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Choose the modal: «He's not answering; he ___ be asleep.» (a likely positive deduction)

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Complete with must/can't/might: «The telescope shows no clouds, so the sky ___ be clear tonight.»

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Write three deductions about a mystery object in the sky using must, can't and might.

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