When permits for the discharge of chemicals into a waterway

The best way to find out an assumption is to negate it.

Lets negate B.
We get -
The flow of water is not enough to "disperse" the chemicals.

But the important point to note is that the argument is based on "dilution" not "dispersion". A chemical can be diluted in stagnant water. Can't it ?
So, even if you negate Option B the argument still stands.

Now, lets negate A.
We get -
The chemicals interact with each other and create other harmful chemicals.

Even if there is flow of water, or even if the chemicals are diluted, if the chemicals can potentially interact to form harmful compounds then the argument falls apart. Doesn't it ?

CONCLUSION: "The waterway is therefore protected against being adversely affected by CHEMICALS under the permits."

(A) Relatively harmless chemicals do not interact with each other in the water to form harmful compounds
What if the discharged chemicals allowed by the permits are SEPARATELY safe for the waterway but can produce COMBINATIONS/COMPOUNDS harmful to it?
Hence, (A) excludes this possibility. Negation of (A) will make the Conclusion break down.
Keep (A)

(B) There is a SWIFT FLOW OF WATER in the waterway that ensures rapid dispersion of chemicals discharged
Notice that it is the AMOUNT of water flowing through the waterway, not the SPEED, that produces the dilution effect of the chemicals. Eliminate (B).

(C) There are no chemicals for which discharge into waterways is entirely prohibited
(C) does not address the logical gap between the Conclusion and the Premises. Try negating (C): There are SOME chemicals for which discharge into waterways is entirely prohibited. This will not affect the Conclusion. Eliminate (C).

(D) Those who receive the permits do NOT ALWAYS discharge the ENTIRE quantity of chemicals that the permits allow
(D) only strengthens the Conclusion: the quantity of discharged chemicals is even LOWER than is allowed by the permits. Negation of (D) "Those who receive the permits ALWAYS discharge the ENTIRE quantity the permits ALLOW" will not make the Conclusion fail because the quantity of each discharged chemical allowed by the permits is ALREADY CALCULATED so that each chemical will NOT harm the waterway. Eliminate (D)

(E) The danger of chemical pollution of waterways is to be evaluated in terms of human health only and not in terms of the health of both human beings and wildlife
The conclusion is concerned with whether the permits ensure that chemicals do NOT affect adversely the waterway IN GENERAL. Whether the danger is in terms of human health only or to both human beings and wildlife is relevant. Eliminate (E).

--> (A) is correct.

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