选词填空

　　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(术语,说法)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(商人)and pilot, Kenneth Arnold, 　　6　　 near Mount(山)Rainier, Washington when he saw nine shiny, round objects(物体)flying through the air(天空)．He told a newspaper reporter that they 　　7　　 like "pie plates skipping(跳)over(在……上方)water"．The reporter used the words "flying saucer(飞碟)" 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．