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I'm struggling with conditional Justifty the Conclusion questions.

For example:Question #12, section 3 of the 2001 December LSAT "Vague Laws", i'm having difficulty understanding why I would pick A over C or E, which are deemed mistaken reversals.

Could someone maybe explain justify the conclusion conditional questions to help me understand how to do it? What would be the sufficient and necessary conditions in the answer?

I have even more toruble with Question 10 of Preptest 31, June 2000 section 2.
COuld someone explain why b is wrong and D is right? I don't understand how to decide whether Justifiably regretted should be sufficent or foregone pleasure should.

For PT 31 Q10 from Sec 2, I think its easier to take a diagrammed approach.

1st sentence: Justifiably regretted (JR) then not desired (~D)....JR---> ~D
2nd sentence (conclusion): Many forgone pleasures (FP) should not have been desired (~D)....FP some ~D.

So,

JR--->~D
----------
FP some ~D

Missing link? It would be between FP and JR..in order to get to ~D. So, FP some JR.

This is perfectly stated in answer choice D. We get, FP some JR--->~D...so it can be concluded FP some ~D.
B states (FP-->~D)-->~JR ....theres no way that will get us to the conclusion. Plus, it almost seems like this a negation of the correct answer.