A minor home remodeling project turns into a major project following Gus's discover there is mold hidden inside the wall. It is important that Gus determine the source of the moisture that caused this mold, because his unendorsed ISO homeowners policy provid coverage for hidden mold resulting from $a(n)$

$($Search Chapter $8)$

a$.$ defective downspout.

b$.$ leaky roof drain.

c$.$ mold endorsement.