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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?
| Text from Passage | Analysis |
| A photograph of the night sky was taken with the camera shutter open for an extended period. |
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| The normal motion of stars across the sky caused the images of the stars in the photograph to appear as streaks. |
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| However, one bright spot was not streaked. |
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| 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 ______. |
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The argument starts by describing a photography setup, then explains what normally happens (stars streak), introduces an unusual observation (one spot didn't streak), and finally challenges the assumption that this non-streaked spot wasn't moving by setting up a claim that needs supporting reasoning.
A celestial object could have been moving across the sky even though it didn't appear streaked in the long-exposure photograph.
The argument uses the setup of normal vs. unusual photographic results to challenge our assumptions about what the unusual result means. It accepts the astronomers' identification of the spot as a celestial object but argues against the implicit assumption that non-streaking equals non-movement.
Logically Completes - We need to find a statement that logically fills in the blank and completes the author's reasoning about why a celestial object could be moving even though it didn't appear streaked in the photograph.
The key claim is about the relationship between movement and streaking in long-exposure photography. The author argues that lack of streaking doesn't necessarily mean lack of movement, so we need to identify conditions where movement wouldn't create streaks.
For this logically completes question, we need to find reasons why a moving celestial object might not create streaks in a long-exposure photograph. We should think about the mechanics of how streaks form - they happen when objects move across the sky relative to the camera's field of view. So we need scenarios where movement exists but doesn't translate into visible streaking.
This choice about the spot not being the brightest object is completely irrelevant to the argument. The brightness level of the spot compared to other objects has nothing to do with why a moving celestial object might not appear streaked. The argument is about movement and streaking patterns, not about relative brightness levels among objects in the photograph.
This choice about streaks caused by non-celestial objects doesn't help complete the argument. We're trying to explain why a celestial object could be moving without creating streaks. Information about non-celestial objects creating streaks is beside the point and doesn't address the core question of how movement can occur without visible streaking.
This choice about stars not shifting position relative to each other actually works against the argument. If stars don't move relative to each other, this would suggest they're all moving together, which doesn't explain why one particular spot wouldn't streak while others do. This doesn't provide a logical reason for why a moving object might not appear streaked.
This choice provides the perfect logical completion. If an object emitted light for only a brief fraction of the exposure time (like a flash), it would appear as a single spot regardless of whether it was moving during the rest of the exposure period. The key insight is that streaking requires continuous light emission during movement - if the light emission is brief enough, no streak pattern can form even if the object is moving. This directly explains how a celestial object could be moving without appearing streaked.
This choice about recording fewer objects with shorter exposure time is irrelevant to the argument. We're not discussing the number of objects captured or comparing different exposure times. The focus is specifically on why one particular bright spot might not show streaking despite potential movement during the actual long exposure that was taken.