A photograph of the night sky was taken with the camera shutter open

IMO D
A sudden flash could be similar to an object which is not moving , hence the streak is absent.
option c although true should not prevent streaks from happening as then other stars would also not produce streaks. The fact that other stars are producing streaks even though they may not be moving relative to each other , shows that this cannot be the reason for that one bright spot not to be producing a streak, which may also be not moving relative to other stars.
IMO D

Straight D - 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

WHy C is wrong someone explain that

If the shutter speed is slow, it can create an effect called “motion blur”, where moving objects appear blurred along the direction of the motion or appear as streaks as stars in this question. The celestial object not necessarily a star may have emitted a flash only for a fraction of the time when the shutter of the camera was open to capture light. Thus D logically completes the argument.

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 __________.

Type - Explain

A. the spot was not the brightest object in the photograph - Incorrect - it does not explain the situation

B. the photograph contains many streaks that astronomers can identify as caused by noncelestial objects - Incorrect - it does not explain the situation

C. stars in the night sky do not appear to shift position relative to each other - Irrelevant

D. 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 - Correct - although it was a celestial object , it did not cause a streak as it emitted light for a short duration

E. if the camera shutter had not been open for an extended period, it would have recorded substantially fewer celestial objects - Incorrect - it does not explain the situation

Answer D

Which of the following most logically completes the argument?

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 __________.

A. the spot was not the brightest object in the photograph - Wrong: Out of scope

B. the photograph contains many streaks that astronomers can identify as caused by noncelestial objects -Wrong: Out of scope

C. stars in the night sky do not appear to shift position relative to each other - Wrong: Shell game ("relative to each other")

D. 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 - Correct: when cause (flash) occurs, effect (spot) occurs

E. if the camera shutter had not been open for an extended period, it would have recorded substantially fewer celestial objects - Wrong: Out of scope

Think of how two cars moving on the highway going in the same direction appear not to shift position relative to each other, but in reality they are both in motion.

Similarly C is the trap answer for the pre-thinking that the celestial object which marks the bright spot might might be a star that does not shift position relative to the Earth. But instead of mentioning the Earth, C mentions stars relative to each other. Hence, it's incorrect as that's not what's happening in our scenario where the camera is on the Earth.

Which of the following most logically completes the argument?

Argument Analysis:
1. A photograph of the night sky taken with camera shutter open for an extended period.
2. The normal motion of stars across the sky caused the images of the stars in the photograph to appear as streaks (Long thin line...) Which means that the camera shutter kept capturing the light that was available during the whole duration of the open shutter, or may be the longer the light was available, the longer the streak became.
3. Now author brings the contrast here and says that "One Bright spot wasn't a streak"
4. Even though the astronomer believe that the spot was because of the celestial object, and that object could still have been moving across the sky during the time the shutter was open.(Then what makes that celestial object different form other stars, which appeared as streaks. Why didn't this celestial body didn't appear as streak?)
We need to account for some difference between both of these celestial bodies to give a reason for the same...
Even if it wasn't the brightest object in the photograph, if it had the normal motion of the star in the sky and a constant light, it should have appeared as a streak and not as a spot. This option doesn't account for the difference.
1. Non celestial objects aren't the matter of concern here in the argument
2. Also we aren't concerned about the streaks formed by the non-celestial/celestial objects, but rather with the spot in the photograph.
This no where makes any connection with the overall argument. Weather stars appear to shift portion to each other or not is not of concerns here. If the stars shift positions to each other then the relative streaks shall be seen as changing positions relative to each other. Doesn't addresses anything with the argument.
This absolutely seems to hit the target. If there is an object that appeared for a fraction of time that the camera shutter was open, the camera made the image for only that particular time and for the rest of the time the camera saw dark image in that place, here only a spot got formed in the image. This explains the spot formation clearly. Let's keep D
Would it record fewer or many celestial objects isn't something that deals in anyway with our concern of the bright spot mentioned in the argument. Goes Out.
D is the best option available...

Official Answer:

Argument Construction

Situation In a photograph of the night sky taken with the camera shutter open for an extended period, the images of stars appeared as streaks because of the stars' normal motion across the sky, but one bright spot was not streaked.

Reasoning What would most strongly suggest that a celestial object moving across the sky could have caused the spot? An object moving across the sky that was bright throughout the time the camera shutter was open should have appeared as a streak in the photograph, just as the stars did. But if the moving object was bright for only a very brief moment, and thus not for an extended time while the camera shutter was open, the object's movement may not have been captured in the photograph, and thus would appear in the photograph as an unstreaked bright spot.


The argument is not about how bright the spot was compared to other objects in the photograph.



Streaks caused by noncelestial objects such as satellites or airplanes do not explain how only one of many celestial objects moving across the sky could have produced the unstreaked spot.



The passage indicates that the stars were shifting position relative to the camera, not relative to one another. In any case, this observation does not help to explain how a celestial object that may not have been a star but that was moving across the sky could have produced the unstreaked spot in the photograph.



Option D is Correct. As explained above, a moving celestial object that only produced a momentary flash of light would produce an unstreaked bright spot in the photograph.



This may be true, given that fewer celestial objects might have moved into the camera's range of view if the camera shutter had not been open as long. But it does not provide any evidence that a moving celestial object could have produced the unstreaked spot.

I got this incorrect by selecting the trap answer C, which suggests that the stars were shifting relative to each other, not the camera.
I overlooked (d) because of the excessive verbiage as well.

Conclusion: Even if the object were a celestial object, that object could still have been moving....
Why? (Premise):.......................

Now look at this photo https://pictures.digitalrev.com/image/upload/w_1920,c_fill/v1432523673/biowgedin4hvphiszndp.jpg

Notice the streaked lights and unstreaked lights. The objects that have no streaks are not moving or are moving very slowly (look at the cars parked in traffic).
Here's the logic behind exposure photograph: moving objects with consistent lighting will streak in photos, but stationary objects (even objects with consistent lighting) will not streak.
So applying this logic to our answers, it's easier to see how (d) is the answer.

The gist of (d): an object could still be moving but since it's not a star, it may not be emitting light the entire time.

So let's complete the argument:
Conclusion: Even if the object were a celestial object, that object could still have been moving....
Why? (Premise):...since that object may not be emitting light consistently like stars do

(A) the spot was not the brightest object in the photograph. Does not really matter as we are not concerned whether it is the brightest or not we need something that explains why this bright spot have happened.

(B) the photograph contains many streaks that astronomers can identify as caused by noncelestial objects. Same as A.

(C) stars in the night sky do not appear to shift position relative to each other. Does not explain how the bright spot have happened.

(D) 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. This gives an explanation to why the bright spot have happened.

(E) if the camera shutter had not been open for an extended period, it would have recorded substantially fewer celestial objects. Does not explain how the bright spot have happened.

The bright spot wast streaked as the camera's cause such spots. The answer is D.

Which of the following most logically completes the argument?

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 __________.

P : stars in the photograph appeared as streaks.
P : one bright spot was not streaked
C : However, the celestial object, which caused spot in the photograph, could still have been moving because ~
Maybe the object's celestial movement was highly synchronous to the speed that earth rotates, causing to be seen as a spot.
Maybe the object only emitted light for a single time while the photograph was taking images.

A. the spot was not the brightest object in the photograph
-> Irrelevant. Being the brightest star has nothing to do in determining the way it is seen.

B. the photograph contains many streaks that astronomers can identify as caused by noncelestial objects
-> Irrelevant. The conclusion is limiting the scope into a celestial object.

C. stars in the night sky do not appear to shift position relative to each other
-> Irrelevant.

D. 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
-> Correct. Because the flash was emitted for only a fraction of time while other starts did emit continuously, the celestial object could be seen as a spot.

E. if the camera shutter had not been open for an extended period, it would have recorded substantially fewer celestial objects
-> Irrelevant. The conditional statement 'if' makes us to think of another situation. However, such situation is not what the argument is mainly talking about.

GMATNinja VeritasKarishma

Please check my reasoning to eliminate C


The stars do not appear to shift position relative to each other, this means that if one of them is a streak, all of them should be.

If the object is a celestial object as the astronomers believe, then this OA becomes irrelevant.

sayantanc2k MartyTargetTestPrep AjiteshArun GraceSCKao

While I agree D is the answer - shouldnt the GMAT give more information about the phrase in option D - "fraction of the time that the camera shutter was open".

Is the fraction - 1 % or 30 % or 80 % ?

If the object emitted a flash for 80 % of the time the camera shutter was open --> wouldnt the object ALSO create a streak, just like the stars did ?

Hi jabhatta2,

Thank you for your question! This CR question was new to me, so thanks for your question, I got a chance to practice it.

I got this question correctly by elimination, so I don't think I can elaborate it well. I did not find the conclusion "that object could still have been moving across the sky during the time the shutter was open" surprising at all, because I think that since Earth rotates on its own axis, it is natural that celestial objects move across the sky. (I know that we need to address the link between the premise and the conclusion, instead of the conclusion itself, but I just tend to check the conclusion at first.) Meanwhile, I do not like the repeated use of "object" in the option (D). But on the other hand, the option (D) would become too obvious if it just said that the spot could have been caused by a flash that disappeared soon.

For the word "fraction," I think in most verbal cases, it refers to "a small part of something," while in the quant section, it refers to "a number that results from dividing one whole number by another," such as 3/5. So for this CR question, I think that "fraction" means a ratio less than 10 percent (maybe 10 percent is even too high.) So, 80 percent mentioned in your example would not be a likely figure. Another evidence is that the author says only a fraction of the time--"only" is usually used to introduce very few of something or low number.

Goal: give a reason why the spot was just a spot when it could have been streaked

C stars in the night sky do not appear to shift position relative to each other -> if they do not shift relatively it means they move all together which all the more says it should have streaked

D 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 -> Here, it clearly means that because the object gave out light only for a fraction of the time that the camera shutter was open then it can be just like a spot

Although this is not completely convincing it is the best option in the given options in my humble opinion!