Critical Reasoning

Every year many people become ill because of airborne mold spores in their homes. After someone becomes ill, specialists are often hired to eradicate the mold. These specialists look in damp areas of the house, since mold is almost always found in places where there is substantial moisture. If one wishes to avoid mold poisoning, then, one should make sure to keep all internal plumbing in good condition to prevent leakage that could serve as a breeding ground for mold.

Which of the following is an assumption on which the argument depends?
A) Mold itself does not create moisture.
B) Most homeowners know enough about plumbing to determine whether theirs is in good condition.
C) Mold cannot grow in dry areas.
D) No varieties of mold are harmless.
E) Mold spores cannot be filtered from the air.

Here the OA is A
Actually I could narrow down my choices to A & C
But then I followed the method given by Powerscore CR bible.
I found out the conclusion.
Here conclusion is "If one wishes to avoid mold poisoning, then, one should make sure to keep all internal plumbing in good condition to prevent leakage that could serve as a breeding ground for mold."
So I thought C would be the best choice.
If we negate C, then conclusion doesn't hold any water.
But if we negate A conclusion is still valid.
Is A the answer because the the question stem says that "Which of the following is an assumption on which the argument depends?"
What if the question would have been like this "Which of the following is an assumption on which the conclusion depends?"