Read the passage. Fill in the gap with one suitable word

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Read the passage. Fill in the gap with one suitable word.
A UFO (Unidentified Flying Object) is any object (1)
which can be unidentified by the person who (2) ..
is investigated. If people cannot figure (3).....
in the sky
.it. Sometimes the object
what the object is after an
investigation, it is called a UFO. If they figure out what the object is, it can no (4)
be called a UFO because it has been identified. Even though UFOs can
be anything, people can (5) .....
aliens' spacecraft. Flying saucer is another (6)
describe an identified flying (7) ...
the word UFO when they are talking about
.... that is often used to
Studies estimate that 50-90% of all
reported sightings are identified later. Usually 10-20% are never identified. Studies also (8)
that very few UFO sightings are hoaxes (people trying to trick other
people). Most UFOs are actually natural or man-made objects that (9)
strange. 80-90% of UFOs are identified as one of these different things: (a) astronomical
causes (for example: planets, stars, or meteors); (b) aircraft; and (c) balloons. 10-20% of
UFOs are other causes, (10) ....
as birds, clouds, mirages, searchlights, etc.