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There have been many UFO _____________reported.

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

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Mercury .................. after the Roman God Mercury.

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

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Mercury is very close to the sun, and it is very hot. Therefore, there .................. not be life on it.

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An astronaut has to wear a spacesuit ___________it may be very cold in space.

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The UFO ………. on a grassy area.

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Scientists have been trying to find ___________ of aliens.