Thanks for the discussion everyone.
Your confusion is understandable since the question was actually copied incorrectly!
In our materials, answer choice A reads "(A) Airport metal detectors, when set to LOWER levels of sensitivity..."
And then the answer is clearly A (though C does remain a strong second-best option). Here's the whole explanation.
We are given a situation in which airport metal detectors are said to be too sensitive, forcing innocent passengers carrying coins to be stopped. The argument states that innocent passengers would be stopped less often (because the small bits of harmless metal they are carrying would not set off detectors) with no loss of security (because weapons are not always metal, and if they are, use a larger amount of metal than a coin) if the detectors were programmed to be less sensitive. The correct answer will be additional evidence that supports the first claim without undermining the second.
Choice A: This choice is correct. This choice bolsters the claim that passengers, presumably innocent, with small bits of metal that are not weapons (watches, belts or jean rivets) will not be stopped unnecessarily. Although this choice adds nothing to the claim that security will retain its ability to screen for weapons, it does not undermine it either.
Choice B: The cost of the proposed plan is not at issue. The desired effect of the plan is to retain security and reduce innocent passenger stops, and this choice does not effect either of those issues.
Choice C: This is a very tempting answer choice, because it is a positive aspect of low-sensitivity metal detectors, but this choice does not, in fact, directly address the claim that fewer innocent people will be stopped by the low-sensitivity detectors. This choice only applies to people already carrying weapons, who are not, by definition, innocent.
Choice D: This choice undermines the claim. If passengers who are stopped for having small amounts of metal are carrying weapons, then reducing security's ability to stop them would hamper the ability to screen for weapons.
Choice E: This choice also undermines the proposal. If only small pieces of metal are used in weapons, then a metal detector must be able to detect small pieces of metal to detect weapons.
But the conclusion has 2 points:
- Reduce the frequency with which innocent passengers are stopped.
- Not to hamper security’s ability to screen for weapons.
A addresses the first point, but not the second one; and C vice versa.
So, how you can say that A is better than C?
The sky is the limit
800 is the limit