Read the passage below and choose one correct answer for each question.

A UFO (Unidentified Flying Object) is any object flying in the sky which cannot be identified by the person who sees it. Sometimes the object is investigated. If people can still not figure out what the object is after an investigation, it is called a UFO. If they figure out what the object is, it can no longer be called a UFO because it has been identified.

Even though UFOs can be anything, people often use the word UFO when they are talking about alien spacecraft. Flying saucer is another word that is often used to describe an unidentified flying object.

Studies estimate that 50%-90% of all reported UFO sightings are identified later.  Usually 10%-20% are never identified. Studies also show that very few UFO sightings are hoaxes (people trying to trick other people). Most UFOs are actually natural or man-made objects that looked strange. 80%-90% UFOs are identified as one of three different things:astronomical causes (for example: planets, stars, or meteors); aircraft; balloons; 10%-20% of UFOs are other causes (such as birds, clouds, mirages, searchlights, etc.)

What does the word “figure out” in paragraph 1 mean?

• A. 
  observe
• B. 
  explain
• C. 
  calculate
• D. 
  require