Here's my understanding of #21 about why choosing A
First, We know that gambling laws are impossible to enforce
Second, we know that a law fails to be effective, it should not be a law
which means Law
So we wanted to prove that the gambling law should not exist, which means
we know that the gambling law is unenforceable, and how can we know it is also not effective?
so we want an answer showing the relationship between not effective and unenforceable, which means A is correct
And why B is incorrect?
B said All enforceable laws are effective, and because there's no evidence in the argument mentioned unenforceable laws that are not effective, and we cannot assume this evidence by answer B because then we'll have a reasoning problem.