In 1986, the city of Los Diablos had 20 days on which air pollution reached unhealthful amounts and a smog alert was put into effect. In early 1987, new air pollution control measures were enacted, but the city had smog alerts on 31 days that year and on 39 days the following year. In 1989, however, the number of smog alerts in Los Diablos dropped to sixteen. The main air pollutants in Los Diablos are ozone and carbon monoxide, and since 1986 the levels of both have been monitored by gas spectrography.

Which of the following statements, assuming that each is true, would be LEAST helpful in explaining the air pollution levels in Los Diablos between 1986 and 1989?

(A) The 1987 air pollution control measures enacted in Los Diablos were put into effect in November of 1988.

(B) In December of 1988 a new and far more accurate gas spectrometer was invented.

(C) In February of 1989, the Pollution Control Board of Los Diablos revised the scale used to determine the amount of air pollution considered unhealthful.

(D) In 1988 the mayor of Los Diablos was found to have accepted large campaign donations from local industries and to have exempted those same industries from air pollution control measures.

(E) Excess ozone and carbon monoxide require a minimum of two years to break down naturally in the atmosphere above a given area.
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Please explain.
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Yes D)
All others provide at least some clue for the drop in 1989.

D is going to result in increase of pollution.

OA is B.
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I dont quite see the difference between B and C. These statements both quantify the pollution levels - in B the instrument measures the pollution levels and in C the revised scale re-assesses how the measured population level should be categorized. Both occurs at a similar time (end of the study) which can account for the drastic drop in 89'. They both do not have a direct impact on CAUSING the population levels - but are statements referring to the aftermath or processing of the results...so if B is the answer, why not C?

I know that explanation was really convoluted and hard to undersatnd..sorry!

For me, B is clearly different from C:
We try to analyze steps form statement to influence on explanation.

(B) In December of 1988 a new and far more accurate gas spectrometer was invented.
1. From invention to production (it requires time, certification and so on).
2. New spectrometers have to be bought, installed and enacted.
3. The accuracy means, for example, 8\pm0.1 instead of 8\pm1. It is difficult for me to imagine the situation when the accuracy may lead to change measurements so dramatically.
4. Moreover, In early 1987, new air pollution control measures were enacted, but ...
too many steps and assumptions.


(C) In February of 1989, the Pollution Control Board of Los Diablos revised the scale used to determine the amount of air pollution considered unhealthful.
1. Revision can be dramatical change threshold value and immediately implemented.

Hope this help
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Althogh walky has based his explanation on the 'time' taken by each method to be effective, I have a different explanation to offer:

Consider this simple analogy:

Say the department of transport develops a far more accurate technique (say a software) to estimate the no. of accidents on high no. 21.
Does that software help in the rise or fall of accidents? No. Say even if its accuracy is 100%, still the accidents could be on the rise or fall, irrespective of the measuring technique used along with its accuracy, repeatability etc.

Now lets say that the department rates the highway unsafe, if 70 or more out of every 100 accidents, are fatal. Lets say thats how the highway has been rated (unsafe) from 2000 to 2006; with say 75-80 fatal accidents in all these years. Now, the department decides to change this scale in 2007. It decides that the highway will be considered unsafe, only if there are 90 or more fatal accidents. At the end of 2007, it is found that the highway had 87 fatal accidents (say). But even thogh these are more than the previous years' values, the highway will be considered safe because of the changes in scale.

Thats the difference between B and C.

What say walker?
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