Thanks for your question!
Let's take a look at the argument from a structural perspective:
- Premise: Replacement parts difficult to obtain Purchase new sirens
Premise: Purchase new sirens Public will be safer
Premise: Local company has gone out of business
Conclusion: Public will be safer
The question asks us to Justify the conclusion that the public will be safer. From the first two premises, we know that the public will be safer if replacement parts are difficult to obtain. Will they be? Perhaps. All we know is that the local company from which parts are purchased has gone out of business. For us to justify the conclusion, we need to establish that if the local company has gone out of business, then replacement parts will indeed be difficult to obtain:
Justify: Local company out of business
Replacement parts difficult to obtain
This prephrase agrees with answer choice (D).
Answer choice (E) does not prove that replacement parts will be difficult to obtain: all we know is that they will be less reliable
. It's entirely possible that such parts are relatively easy to obtain; the reliability of the replacements has no bearing on the issue at stake. If you consider answer choice (E) to be a contender, you are assuming that the government will automatically decide to purchase new sirens just because the replacement parts for the old ones will make them less reliable. We have no reason to make this assumption.
Does this make sense? Let me know.