A photograph of the night sky was taken with the camera shutter open for an extended period. The normal motion of stars across the sky caused the images of the stars in the photograph to appear as streaks. However, one bright spot was not streaked. Even if the spot were caused, as astronomers believe, by a celestial object, that object could still have been moving across the sky during the time the shutter was open, since __. Which of the following most logically completes the argument? ['the spot was not the brightest object in the photograph', 'the photograph contains many streaks that astronomers can identify as caused by noncelestial objects', 'the spot could have been caused by an object that emitted a flash that lasted for only a fraction of the time that the camera shutter was open', 'stars in the night sky do not appear to shift position relative to each other']