Self-study
Conclusion It would be wrong to conclude that a person has a Streptococcus infection if there is no other evidence than the fact that Streptococcus bacilli are present in the person's throat; after all, Support infection does not occur unless the host is physically run down.
Two Necessary Conditions, But Only One Is Confirmed Met
The argument's move is "you've only checked one box, and there's another required box you haven't checked." Yes, the bacilli are in the person's throat. But infection also requires the host to be physically run down. Without verifying that second condition, you can't conclude there's an actual infection.
The stimulus identifies a necessary condition for the conclusion (being physically run down) that the available evidence doesn't address. That's the structural feature we want to find in an answer.
Anticipation
We're looking for an argument where the conclusion warns against jumping to a claim because, in addition to the evidence at hand, there's another necessary condition that hasn't been verified. Two separate conditions are required; the available evidence covers only one.
20.
The reasoning in which one ██ ███ █████████ ██ ████ ███████ ██ ███ █████████ ██ ███ ████████ ██████
a
When a person ███████████ ███████ ███████ ██ ████ ███ ██████ ████ █ ████████ ██████ ██ ███ ████████ ████ ██ ███ ██████ █████ ████████ ██ █ ████████ ██████ ████ ███ ██ ███████ ██ ███████ ██████
(A) is alternative-cause reasoning, not necessary-condition reasoning. (A) says we can't conclude eye defects caused the blurred vision because drugs can also cause it (alternative cause). The stimulus isn't saying "rundown is an alternative cause to bacilli." It's saying both bacilli and rundown are required, and we've only verified one. Different logical structure.
b
Even if a ███████ ████████ █████ ████████ ███ ██ ████ █████ ██ ██████ ████████ ████ ████ ███ ██████ ███████ ██ ████ █████ █████ ████ ███ █████ ████ ██████ ███████ ████████ ████████ ████ ███ ██ ████ █████ ██ ████████ ███ ███ ███ ████████ ████████ ████ ██ ██████
This matches the stimulus piece for piece. Blooming requires two conditions (six or more hours of sunlight AND slightly alkaline soil), just as infection requires two conditions (bacilli AND a rundown host). We've verified one of the two in each argument (sunlight in (B), bacilli in the stimulus), but not the other (alkaline soil in (B), rundown in the stimulus). So we can't draw the conclusion (blooming in (B), infection in the stimulus).
c
When a bee ██████ █████ ██ ███████ ███ ███████ ██ █████ ██ █████ ██ ████████ ████ ███ ████████████ ████ ███ ██████ ████ ████ ████ ████ ███████ ██████████ ███████ ███████ █████ █████ ███ ████████ ██ ██████ ████████ ██ ███ ███████ ██████ ███ ██ ██████████ █████
(C)'s move is "undermine the proposed cause," not "second necessary condition not verified." (C) argues we can't conclude low temperatures caused the colony's failure because bees have defense mechanisms against cold (so cold isn't likely to kill them in the first place). The stimulus isn't saying rundown is an unlikely cause of infection because of some defense mechanism. It's saying rundown is required, and we haven't checked for it.
d
A female holly █████ ██████ ███████ ███████ ███████ █ ████ █████ ██████ ██ ███████ ███████ ███ ██ ████ ███ ██████ ████ ███ ██ ████ ████ ███████ ██ ███ ████████ ████ █████ █ ██████ █████ ██ ████ ████ ███████ ████ ██ █████ ████ ████ █ ██████ ████ █████ ███████
(D) is about whether multiplying a necessary condition multiplies the effect. The starting point is that one male holly is necessary for berries; the question is whether two or more males would produce more berries. The stimulus doesn't make any such comparison. It's not asking whether more bacilli or more rundown would mean more infection. It's asking whether a single piece of evidence is enough to conclude infection at all.
e
A person cannot ██ ████████ ██ ██ ████████████ ██ ███ █████ ██ █ ████ ███████ ███ █████ ████████ ████ ██ ███████████ ███ ████ ███████ █████ ████ ██████ ████ ███████████ ████ █████ ████████ ███ ████████ ██████ █████████████
(E) is the most tempting wrong answer, but its structure is different. (E) says we can't conclude hypertension from a single high blood pressure reading because hypertension is defined as chronically high blood pressure. The issue is that the evidence is too brief in duration; we need a sustained version of the same kind of evidence. The stimulus identifies a separate, additional necessary condition (being physically run down) that the available evidence doesn't address at all. (E) needs more of the same condition; the stimulus needs a different condition.