This is the quick solution I came up with:
Conc: Residential sprinklers would only marginally decrease property damage caused by residential fires.
(A)
Most individuals have no formal training in how to extinguish fires. - Can co-exist with the conclusion above. No need to think too much.
Drop(B)
Since new homes are only a tiny percentage of available housing in the city, the new ordinance would be extremely narrow in scope - This actually strengthens the argument. Since, only a small sliver of housing in the city will have the sprinklers and since 90% of fires are extinguished by household members, sprinklers might be an ineffective addition.
Drop(C)
The installation of smoke detectors in new residences costs significantly less than the installation of sprinklers - We are specifically talking about the necessity of sprinklers and preventing property damage caused by fire. Whether smoke detectors are cheaper or not doesn't impact the conclusion.
Drop(D)
In the city where the ordinance was proposed, the average time required by the fire department to respond to a fire was less than the national average - The speed of response of the fire dept. could be a magnitude better or worse than national average, however, it's still entirely possible that sprinklers are not that effective since they are not needed most of the time. This option actually tries to suggest that sprinklers are not needed, since fire-depts. are very responsive. Not what we're looking for (and not what you want to hear from your fire safety inspector).
Drop(E)
The largest proportion of property damage that results from residential fires is caused by fires that start when no household member is present. Hits the bulls-eye. If the largest proportion of property damage results when no household member is present, the sprinklers look like a welcome addition. Also notice, that the conclusion talks about property damage but substantiates the claims with number of fires extinguished. Those 2 are related, but not the same. This option gives us a reason ,i.e., fires in unattended houses cause the most damage.
Keep