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IMO C. More the distance traveled by the light to reach the theodolites, more the refraction error.
see lines 33-38.

i chose A, because large variations in sea level affects air pressure and temperature.

C seems good to me too, but i felt it may be a little out of scope. What the heck is the OA?

I agree with C.
As when the theodolite is used relatively far from the mountain peak -> more chance to have temperature and pressure changes -> most likely to cause the mentioned error.

Tough one.
A is out of scope. Local variation is not the cause of refraction' error
B Tricky. Light passed through the air with different temperature and pressure. Does not specifically mention humid. Eliminated
C hold on
D launch balling to measure the Temperature and pressure change to reduce the refraction error. It means they can measure when it is low temperature and pressure.eliminated.
E out of scope.
If it is far relatively far for the peak, it increases the chance of refraction' error. Answer is C

I agree with C as it seems to be the most obvious answer
Where is this question from? Please post the OA

I vouch for C, as mentioned in the passage refraction would create error in cases where we have differences in temperature and pressure; thus we need to look at choice that implies such variations.

A. When there are local variations in sea level : Sea level unevenness leads to another type error thus unrelated. (line 53)
B. When light passes through humid air : humidity is out of scope, was never discussed
C. When theodolites are used relatively far
from the mountain peak. : seems a plausible option as it may signify large variation in T and P with altitude (line 30-35)
D. When weather balloons indicate low air temperature (T) and pressure(P). : Its changes in T and P not their absolute value that results in error
E. When sea level has been carried in to within five to twelve miles of the summit.: irrelevant to the question

I understand (and don't necessarily disagree) why people are saying C, but humid air has both a different temperature and pressure than non-humid air.

If we are able to bridge that gap (from humid to "different temperature and pressure), than why isn't B correct?

C the answer

B.
everbody! humid air that means it's containing water, whenever light go through water with different temperature will have a refraction.

otherchoices are the ways how to correct errors

@vongochao

I think anybody who picks C has chosen the "smart" answer, but unfortunately it's not the "correct" answer since you'd have to make the jump from "humid" to "different temperature (and pressure)", which is just too much of a jump for a GMAT question since it's not in the text.

Answer is C.

Answer: C

We can solve this problem but using logic and confirm using context clues.

B and D end being wrong because humid air or low pressure air does not translate to CHANGING temp/pressure i.e. it could all be the same still. The 2nd problem is that even IF that did mean changing temp/pressure it does not mean that it affects measurements since we could still be measuring at very close distances

The conditions in C allow the most probability to cause errors since, from the passage, we know that greater the measuring distance the more refraction errors because there is more distance and thus more changes in air/pressure. Also, we can feel comfortable because the major fix to refraction errors was to carry in 5-12 miles from the summit