Some plants have extremely sensitive biological thermometers. For example, the leaves of rhododendrons curl when the temperature of the air around them is below 0℃(Celsius). Similarly, mature crocus blossoms open in temperatures above 2℃. So someone who simultaneously observed rhododendrons with uncurled leaves, crocuses with mature but unopened blossoms, and a thermometer showing 1℃ could determine that the thermometer’s reading was accurate to within plus or minus 1℃.
Which one of the following, if true, most seriously undermines the reasoning above?
(A) Neither rhododendrons nor crocuses bloom for more than a few weeks each year, and the blossoms of rhododendrons growing in any area do not appear until at least several weeks after crocuses growing in that area have ceased to bloom.
(B) Many people find it unpleasant to be outdoors for long periods when the temperature is at or about 1℃.
(C) The climate and soil conditions that favor the growth of rhododendrons are also favorable to the growth of crocuses.
(D) Air temperature surrounding rhododendrons, which can grow 12 feet tall, is likely to differ from air temperature surrounding crocuses, which are normally only a few inches high, by more than 1℃, even if the two plants are growing side by side.
(E) Certain types of thermometers that are commonly used to measure outdoor temperatures can be extremely accurate in moderate temperature ranges but much less accurate in warmer or colder temperature range.
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stimulus
the leaves of rhododendrons curl when the temperature of the air around them is below 0℃(Celsius).
mature crocus blossoms open in temperatures above 2℃

conclusion
Someone who simultaneously observed rhododendrons with uncurled leaves, crocuses with mature but unopened blossoms, and a thermometer showing 1℃ could determine that the thermometer’s reading was accurate to within plus or minus 1℃.

to weaken the conclusion we have to show that the temperature shown by the thermometer is not accurate
D correctly points out that the surrounding temp of rhododendrons can differ from the surrounding temp of crocuses by more than 1℃.

The answer should be D. It cannot be A as in that, they are talking about blooming of rhododendrons which is not the case. Whether they bloom or not, the leaves will be present when the crocuses bloom.
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Re-taking GMAT. Hope the charm works this time..

I went for D, what's the OA?

Maybe true, but out of scope (A) Neither rhododendrons nor crocuses bloom for more than a few weeks each year, and the blossoms of rhododendrons growing in any area do not appear until at least several weeks after crocuses growing in that area have ceased to bloom.

Irrelevant (B) Many people find it unpleasant to be outdoors for long periods when the temperature is at or about 1℃.

Irrelevant (C) The climate and soil conditions that favor the growth of rhododendrons are also favorable to the growth of crocuses.

Correct. The height difference between the 2 can definitely result in different temperatures that they are exposed to. The only way that makes any sense to undermine the statement (D) Air temperature surrounding rhododendrons, which can grow 12 feet tall, is likely to differ from air temperature surrounding crocuses, which are normally only a few inches high, by more than 1℃, even if the two plants are growing side by side.

Maybe true, but irrelevant (E) Certain types of thermometers that are commonly used to measure outdoor temperatures can be extremely accurate in moderate temperature ranges but much less accurate in warmer or colder temperature range.
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D

Can you share the OA please?
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Its your duty to post OA afterwards; some one must be waiting for that...

+1 D

Somebody in 2005 on other forum said the OA is D.
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