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#25 - At one time, many astronomers assumed that Earth

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Complete Question Explanation

Assumption—SN. The correct answer choice is (D)

As we saw with question 20, this is an Assumption question in which conditional reasoning plays a dominant role.
The author of this stimulus describes some erroneous ideas many astronomers once held about the relationship between Earth and the stars. At one time, many astronomers assumed that Earth remains motionless while the stars revolve around it. Based on this assumption, the astronomers concluded that the stars could not be more than a few million miles from Earth.

The astronomers reached this conclusion after considering that if the stars were farther away from Earth, they would have to move at tremendously great speeds in order to circle Earth during the day and reappear in roughly the same positions each night. This conditional relationship can be diagrammed as:

FA = stars farther away f rom Earth than a few million miles
TGS = stars move at tremendously great speeds

    FA ..... :arrow: ..... TGS

Recall the astronomers’ conclusion that stars were not more than a few million miles from Earth. In keeping with the terms we just used in the conditional relationship above, their conclusion would be symbolized as “FA”. This should prompt you to consider the contrapositive of that relationship:

    TGS ..... :arrow: ..... FA

Since the stimulus provides no other evidence to support the astronomers’ conclusion that the stars were not more than a few million miles from Earth, it must be the case that the astronomers assumed that the stars are not capable of moving at tremendously great speeds (i.e., TGS).

Answer choice (A): This answer choice is irrelevant to the conclusion, because the assumption expressly provided by the stimulus as underlying the astronomer’s argument was that Earth remains motionless while the stars revolve around it.

Answer choice (B): The stimulus dealt only with the speed of the stars in terms of their positions relative to Earth, and not with the speed of stars relative to each other.

Answer choice (C): This answer choice is inconsistent with the stimulus, which expressly stated the astronomers reached their conclusion based, in part, on their belief that Earth remains motionless.

Answer choice (D): This is the correct answer choice. As described above, this information is required for the conclusion to be valid. If stars do move at tremendously great speeds, then there would be no support for the astronomers’ conclusion regarding the stars’ distances from Earth.

Answer choice (E): This answer choice is tricky. While it explicitly references the distance of “a million miles,” it implicitly relates not only to stars just over a million miles from Earth, but also any star at any distance “more than a million miles from Earth.” So, for this answer choice to be correct, it would be required for the astronomers’ conclusion that a star one billion, or even one trillion miles from Earth could reappear in roughly the same position each night. While it is clear from the stimulus that the astronomers believed a star only a few million miles from Earth could reappear in roughly the same position each night, when taken to its extreme, this answer choice is actually inconsistent with the astronomers’ conclusion, rather than being required by it.