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The whole basis of the conclusion is " if one wishes to avoid mold, then, one should make sure to keep all internal plumbing to prevent leakage....". This statement says, if you do not have water or any moisture in your house due to leaks or anything else, you will not grow mold. The assumption here is that you get mold from leaks, plumbing and wet sources, not from actual mold. If we found out that mold creates moisture, then his argument would make no sense because it doesn't matter whether you get a leak or not, the mold is the one that causes to moisture. Hope this helps.

I understand that A is an assumption, but why is C not an assumption.

If mold could grow in dry areas, then how could one avoid mold by keeping all internal plumbing to prevent leakage?

Hi Thank you, i read your explanation. you state the following reason why C cannot be the answer:

From the passage: Mold is almost always found in places where there is substantial moisture.
This statement is a PREMISE: a fact not in dispute.
ALMOST ALWAYS means NOT ALWAYS.
Thus, the passage states -- as a FACT -- that mold is NOT ALWAYS found in areas where there is substantial moisture.
In other words, mold CAN be found in dry areas.
Answer choice C seems to refute this premise.
The correct answer to a CR can NEVER refute a premise.

Why does this mean C is refuting a premise? The premise doesn't seem to state that mold GROWS in dry areas, only that it might. For example, what if NOT ALWAYS found in substantial moisture means that Mold is sometimes found in areas that simply does not have substantial moisture, not that it is dry, but maybe a normal moisture content, or perhaps just higher than normal moistness, even very moist, so long as it is not "substantial". Although very seems synonymous to substantial, perhaps very would not work, but all other degrees of moistness could be ok.

Also could you comment on why it seems that the negate technique works better for C than for A?

if moisture can be found in dry areas, then avoiding mold by preventing leaks is weakened.

if moisture itself DOES create moisture, then preventing leakage is weakened as well, but since specialists are hired to eradicate mold, no leakage + no mold = no future mold.