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Stimulus:

~ENFORCE

~EFFECTIVE ---> ~LAW
-------------------------------------

~LAW


Both answers (A) and (B) seem to work to bring us to the conclusion of ~LAW

A) No effective law is unenforceable.

EFFECTIVE ---> ENFORCE

Thus, ~ENFORCE ---> ~EFFECTIVE

This contrapositive can get me to the conclusion of ~LAW


B) All enforceable laws are effective.

ENFORCE ---> EFFECTIVE

Thus, ~EFFECTIVE ---> ~ENFORCE

This contrapositive cannot be used validly to get to the conclusion of ~LAW because of the different necessary conditions? I want to say that I have a case of ~EFFECTIVE, so I can then link it with the second premise of the argument to get to ~LAW.

My answer may not be helpful because it is more intuitive than mechanical but the passage says nothing about all enforceable laws being effective. The conclusion is that "there should be no legal prohibition against gambling" because a. the laws are impossible to enforce and b. when a law fails to be effective, it should not be a law.

In order for the conclusion to be drawn, one must assume that no effective law is unenforceable (answer A). All effective laws being effective (answer B) does not address the conclusion that there should be no legal prohibition against gambling because the laws are unenforceable and thus fail to be effective and thus, should not be a law.

~enforceable --> ~effective

The contrapositive is:

Effective --> Enforceable (which is what answer A says in other words)

Answer B says enforceable --> effective (which fails to switch the variables)

~A --> ~B
B--> A (answer A)

not necessarily if A then B (answer B)

HTH

I'll give this a try using the mechanical approach...

Since the conlusion we want to draw is ~LAW, (B) can't be correct. This is what happens when we link (B) to the argument:

~EFFECTIVE --> ~ENFORCE and ~LAW
We have concluded two conditions 1)~ENFORCE, 2)~LAW; however, the conclusion we want to draw only has one condition (~LAW).

Now let's try (A):

~ENFORCE --> ~EFFECTIVE --> ~LAW
This is exactly what we want to conclude.

While (B) allows us to conlude ~LAW, it goes too far because it includes another condition.