This is from a Manhattan GMAT CAT (my 3rd one) I took about a week ago:

Before the age of space exploration, the size and composition of the moon's core were astronomical mysteries. Astronomers assumed that the moon's core was smaller than that of the Earth, in both relative and absolute terms -- the radius of the Earth's core is 55 percent of the overall radius of the Earth and the core's mass is 32 percent of the Earth's overall mass -- but they had no way to verify their assumption. However, data gathered by Lunar Prospector have now given astronomers the ability to determine that the moon's core accounts for 20 percent of the moon's radius and for a mere 2 percent of its overall mass.

The data have been used in two ways. In the first method, scientists measured minute variations in radio signals from Lunar Prospector as the craft moved towards or away from the Earth. These variations allowed scientists to detect even the slightest changes in the craft's velocity while the craft orbited the moon. These changes resulted from inconsistency in the gravitational pull of the moon on the craft, and permitted scientists to create a "gravity map" of both near and far sides of the moon. This map, in turn, revealed to scientists the distribution of the moon's internal mass. Scientists were then able to determine that the moon has a small, metallic core, which, if composed mostly of iron, has a radius of approximately 350 kilometers. The second method involved examining the faint magnetic field generated within the moon itself by the moon's monthly passage through the tail of the Earth's magnetosphere. This approach confirmed the results obtained through examination of the gravity map.

The size and composition of the moon's core are not academic concerns; they have serious implications for our understanding of the moon's origins. For example, if the moon and Earth developed as distinct entities, the sizes of their cores should be more comparable. In actuality, it seems that the moon was once part of the Earth and broke away at an early stage in the Earth's evolution, perhaps as the result of a major asteroid impact. The impact could have loosened iron that had not already sunk to the core of the Earth, allowing it to form the core around which the moon eventually coalesced.

1. The primary purpose of the passage is best expressed as

A. The author discusses the method of data utilization that proved that the moon was once part of the earth.
B. The author discusses the methods of data utilization that changed scientists' opinions about the relative sizes of the earth's and moon's cores.
C. The passage studies the method that, using data about radii and core mass, led to important implications about the origin of the planets.
D. The passage studies the use of scientific methods to analyze data and confirm a hypothesis and references possible deductions.
E. The passage discusses the implications following from the size of the Earth's core.


2. According to the passage, scientists employed one research method that measured

A. changes in a spacecraft's velocity as it returned to Earth
B. changes in the gravitational pull on the Lunar Prospector as it orbited the moon
C. changes in the moon's gravity as it orbited the earth
D. changes in a spacecraft's radio signals as it changed position relative to the earth
E. changes in the moon's gravitational pull over time

I had two quick questions:

#1 - isn't it a bit of a stretch to describe the primary purpose and not mention anything about the moon? None of the other answers seems correct but D seems to be more of a characterization of the passage, not a description of its primary purpose?

#2 - Isn't B a perfectly supported answer as well?

"These variations allowed scientists to detect even the slightest changes in the craft's velocity while the craft orbited the moon. These changes resulted from inconsistency in the gravitational pull of the moon on the craft, and permitted scientists to create a "gravity map" of both near and far sides of the moon."

If the scientists mapped out the inconsistencies in the gravitational pull on the aircraft, wouldn't that be measuring changes in the gravitational pull?

Thanks much for your help!

oh no. not at all. if you're looking for things like that, you're going to come up empty on pretty much every single "main idea" problem.

ALMOST ALL main idea questions GENERALIZE the main idea of the passage, using terms like "a theory", "a hypothesis", etc., and don't actually mention the specifics.

for instance, if a passage argues that, say, high-speed trains are superior to commuter airplanes for trips of 200 to 500 miles, the correct answer to a main idea question would say something like
main idea = to compare the relative merits of two forms of transportation
or even
to compare two means of achieving the same purpose
it is almost certain that the correct answer will NOT, repeat NOT, mention anything specifically to do with trains or commuter planes.

same deal here.

#70 in OG is a good illustration, although the vast majority of main idea questions will behave similarly.

nope.

you're not MEASURING gravitational pull.

per the passage, you are measuring changes in velocity, and using those to INFER facts about gravitational pull.

analogy: let's say i look at cracks in the sidewalk and use them to infer the location of earthquake faults.
it would be correct to say i'm observing cracks in the sidewalk (analogous to "measuring").
it would be correct to say i'm inferring the location of earthquake faults.

it would be totally wrong, however, to say that i'm observing the location of earthquake faults. see the difference?

1.
The author most likely mentioned "gravity map" in the second paragraph in order to

A: illustrate the method used to correct the Lunar Prospector's velocity
B: confirm the results obtained by examining the Moon's magnetic field
C: detail a step in the investigation of the composition of a specific orbiting body
D: detail a step in the investigation of the effects of gravity on the Moon and the Earth
E: explain a step in the process of confirming a new hypothesis and discarding an older theory

OA: C

Yes, I also agree but in C, "orbiting body" is refereed to Moon. Based on this passage, how can I know that Moon is a orbiting body.

Above answer lies in:-
The data were used to create a "gravity map" of both near and far sides of the Moon, highlighting new details of the distribution of the Moon's internal mass.


2.
The author most probably uses the word “so-called” (line 53) to imply that
A: the force may not have been adequate to cause the claimed ejection
B: fission theory may have less explanatory power than impact theory
C: the later coalescence of the Moon’s material may not have occurred as a result of centrifugal force
D: the speed of the early Earth’s rotation may have led to the creation of the Moon
E: the phenomenon in question might more properly be re-named

OA: E

I selected D but this is not OA. I could not convinced from OE.

It is in:-
Alternatively, according to fission theory, the early Earth may have spun so rapidly that it ejected a quantity of material by so-called centrifugal force, material that later coalesced by mutual gravitational attraction into the Moon.

I'm sorry, but I've looked through the passage posted above and don't see the lines you reference so I can't help a lot here. However, in the English language the term "so-called" is generally used to indicate that while an object or phenomenon may have a particular name, that name is a poor fit to describe what is actually going on or existing.

_________________
Jamie Nelson
ManhattanGMAT Instructor