This is a Necessary Assumption question. Could someone explain how Answer B is incorrect and Answer D is correct?
Answer B speaks to the gap (Detergent formulated for front-load dissolves more readily). Negating Answer B (Detergent formulated for front-load does NOT dissolve more readily) wrecks the conclusion because it removes the salient difference compared to ordinary detergent that the argument makes.
In contrast, Answer D provides a Sufficient Assumption to a Necessary Assumption question. Negating Answer D (It's not true that [detergent gets clothes really clean --> detergent dissolves readily in washer]) does not wreck the conclusion. Perhaps a detergent formulated for FLW simply requires less water to be just as effective as an ordinary detergent in a TLW. Or perhaps a detergent formulated for FLW has a special cleaning agent to compensate for not dissolving as readily. So on and so forth: In all cases, Sufficient does not equal Necessary.
Furthermore, working from wrong-to-right I eliminated Answer D as a trap. Answer D provides a "firmer" (conditional guarantee!) response more appropriate to a SA question. In contrast, Answer B provides a "softer" (comparative) response appropriate to most NA questions.
Thanks in advance for your help!
https://7sage.com/lsat_explanations/lsat-70-section-1-question-21/
Comments
ac-B doesn't have to be true because the detergent specially made for front loaders could very well dissolve more readily in a top-loader. In fact that would would make plenty good sense as we are told that there is more water in the top-loader and so it is conceivable that it would dilute even easier. ac-D says that if a detergent does not dissolve readily in a washer, then that detergent does not get clothes really clean. This MUST be true for the conclusion to hold. (the negation of this statement says: if a detergent does not dissolve readily in a washer, it IS still possible that clothes could get really clean.) This contradicts the conclusion.
However, negating Answer D (a Sufficient Assumption) only rules out one way to close the gap in the argument, rather than ruling out the conclusion itself. Note the conclusion is, "you need a detergent especially formulated for FLW to get clothes really clean." What's to say that "especially formulated" doesn't refer to a special cleaning agent to compensate for not dissolving as readily? So on and so forth - because Answer D is a SA (just one way to close the gap) rather than a NA.
Are there other example NA questions for which the CR is a SA?
If it's not true that the detergent needs to dissolve well to get clothes clean, then you don't NEED a specially formulated detergent to get your clothes clean, because an ordinary one, even if it's sitting in clumps at the bottom of the front loader is still somehow doing its job just fine. Maybe the front loaders are super duper efficient at shaking and spinning, so they can get rid of dirt with homeopathic amounts of detergent.
Argument:
P: O --> not DR (contrapositive: DR --> not O)
C: RC --> not O --> S
Answer D:
RC-->DR
O (Ordinary detergent in front-loader)
DR (Dissolves Readily in front-loader)
RC (Really Clean in front-loader)
S (Special detergent in front-loader)
The conclusion introduces two new elements (RC & S) but only RC is linked to the premise, by Answer D (RC --> DR --> not O). The link to S (not O--> S) remains an unsupported gap.
This means another NA could be, for example, "There is no additive to ordinary detergent that would cause it to dissolve readily in a front-loader." Negated, this would lead the argument to another conclusion (i.e., ordinary detergent, in lieu of special detergent, can get clothes clean in a front-loader).
Because Answer D does not completely bridge the gap between the premise and conclusion, it's not an actual SA but rather (correctly) a NA. Well played, LSAC, well played.