I believe that the correct answer is D because it is already stated once that they need the sirens so it is giving us a fact that has already been stated.
I think the answer you're looking for is D. It would make sense that D is the answer (to me at least) because: The preceding Sentences do not repeat each other, but they do stress the need to get the alert system, however the other sentences may not say it in the same way, the last sentence could already be concluded by the reader. (I hope this helps, I'm 75% - 85% sure this answer is right, either D or B, I would lean towards D, but if it comes down to it go with your gut - advice that seems lame that actually helps A LOT)
