We need to strengthen our belief in the researchers’ argument that the drugs in the water are not a significant health hazard. Bridging one of the gaps could be a good way to strengthen.

(A) Incorrect. We need to pay close attention here. We discussed above that no ‘discernible health effects’ does not necessarily mean no health effects. If we are not precise, this option seems to alleviate that concern. However, this option actually reverses the conditional. We are interested in figuring out: if a drug found in water is a health hazard, then will its presence in the water have any discernible health effects?

Basically, on the basis of ‘no discernible health effects’ is it safe to infer ‘no high levels of drugs in water’ and thus ‘not a significant health hazard’? That is how the argument is built. The argument assumes that ‘no discernible health effects’ is sufficient to conclude ‘no significant health hazards’.

What we are given here is: ‘not a significant health hazard’ implies ‘no discernible health effects’. This option changes the direction of the sufficiency. That does not help the argument. 

Here’s an example to understand this further:

Argument: If a person does not have a blue eye, we can conclude that the person has not been in a fight. (relate this with the argument)

Statement: If a person has not been in a fight, the person cannot have a blue eye. (relate this with this option)

Does the above statement strengthen the above argument? 

Nope. Just because ~X (no fight) means ~Y (no blue eye) doesn’t mean that ~Y means ~X. We can easily have ~Y (no blue eye) and X (a fight).A person could have been in a fight with someone much weaker and not got hurt at all. He could have been hurt in other ways. He could have very well still fought. 

The same reasoning applies to this option as well.

How about this statement: If a drug found in drinking water is not a significant public health hazard, then its presence in the water will not have any discernible health effects.

What impact does this statement have on the argument?

(B) Incorrect. Researchers’ argument is that the drugs in the water are not a significant health hazard. Whether we need to remove the drugs from water is beyond their argument. 

(C) Incorrect. Scientists may not have discerned ‘which’ adverse health effects a health hazard has caused. This is not the same as scientists may not have discerned ‘whether’ there were adverse health effects. Even if the scientists were not able to pinpoint which adverse health effect a substance caused they could have still understood ‘whether’ a substance had adverse health effects.

Let us also consider the statement we made above: Scientists may not have discerned (detected) whether there were adverse health effects. Does this statement strengthen the argument? It doesn’t. It actually weakens the argument. This is the second gap we have discussed above.

(D) Correct. So researchers did detect these drugs in the water in another town decades ago. And they could not find any visible side effects. Ok, so these drugs very well may be present in the water and yet there may not be any noticeable effects. That does increase our belief in the researchers’ reasoning. These drugs do not manifest any noticeable health effects, and likely are not a significant public health hazard.

Let’s look at one more aspect. How come researchers were able to detect the drugs ‘several decades ago’ when the passage states that ‘they could only be detected with the most recent technology’? The passage says that the levels were ‘so low’ that only the latest technology could detect the drugs. So, if in a neighboring town researchers could detect the drugs several decades ago with older, less sensitive technology, that indicates that the levels of drugs would have been higher. If even those higher levels did not lead to any discernible health effects, it is likely that even the current low levels would not and thus the drugs in the water are not a significant health hazard.

(E) Incorrect. Wait a minute. Old samples did not have the drugs? That would likely mean that the reason there were no ‘discernible health effects’ is that there were no drugs in the water. The drugs perhaps only recently got added to the water. This option is clearly in the opposite direction, and weakens the argument.

Before we get too engrossed in the nitty-gritty, it helps to understand what ‘direction’ an option is taking our argument in. This one clearly is going in the opposite direction. It reduces our belief in the reasoning and is thus incorrect.