阅读短文，用方框中所给词的正确形式填空。

There is something people didn't understand in the sky. The USAF (美国空军)    1     used the
words"Unidentified Flying Object" to describe them. Today     2    uses the term UFO.  When people
see a UFO, it is    3     a weather balloon (气球) or an airplane. But the Center    4    UFO Studies
continues to  receive    5     of reports every year．
In 1947 a businessman and pilot, Kenneth Arnold.    6     near Mount Rainier, Washington when he
saw nine shiny, round objects flying through the air. He told a newspaper reporter that they   7     like
"pie plates  skipping over water". The reporter used the words "flying saucer" in his-story and today that
    8    a popular name' for UFOs.
Now in different countries, many people    9     the UFOs. Some people make mistakes when they
say they have seen UFOs. But people still can't    10   UFOs clearly. It may take many years to find a
clear answer.

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