Critical Reasoning

I am confused between B and D
Plz explain both options...thanks
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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Had D true, the pollution level should have increased in 1989, opposite of the given premises in the argument. B shows that in 1988 a more accurate spectrometer was invented, facilitating better measurements. B could be a reason for reduction in the number of smog alerts. Hence, IMO D.

In a question like this, it's best to explain all answer choices, and determine which is least likely to help explain the drop in air pollution levels in 1989.

A helps to explain why the drop wasn't until 1989, because the new measure were not put into effect until very late in 1988.

B helps to explain why the drop wasn't until 1989 because in late 1988, a new device was used to measure the amount of smog, and the device by measuring it more accurately might have measured a lesser amount of smog in the atmosphere, resulting in a lesser number of alerts.

C helps to explain why the drop wasn't until 1989, because the new revision in 1989 could have raised the amount of smog necessary for an alert, and therefore decreased the total number of alerts in 1989.

E helps to explain why the drop wasn't until 1989, because in the 2 years prior, the pollutants had not yet broken down naturally, and were still in the atmosphere, causing the increased number of alerts.

D, however, does not help to explain why the drop wasn't until 1989. According to D, the mayor accepting large donations from certain certain industries, and exempted those industries from air pollution control measures. That being the case, with less industries being measured, we would expect to see a decrease in 1988. So this does not at all help to explain why the decrease did not occur until 1989.

I also think D should be the answer