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What _______ when I called?

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Alice _______ the accident when she was catching the bus.

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The prerequisite condition for a/an _______ planet is water supplies, appropriate temperatures and breathable air.

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The scientists believe that there might be some planets to _______ human life.

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What were you doing when you _______ the strange sound?

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Researchers use this large spacecraft as a place to live and work in space.

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The _______ can fall below zero degrees Celsius in winter.