Self-study
Newspaper editor: Law enforcement experts, as well as most citizens, have finally come to recognize that legal prohibitions against gambling all share a common flaw: no matter how diligent the effort, Support the laws are impossible to enforce. ███████ ██████ ████████████████ ████ █ ███ █████ ██ ██ ██████████ ██ ██████ ███ ██ █ ████ ████ ██ ███ █████ ██████ ██ ██ █████ ███████████ ███████ █████████
Summary
The conclusion is that there shouldn’t be a legal prohibition against gambling. This is based on the following:
Gambling laws can’t be enforced.
If a law is not effective, it should not be a law.
Missing Connection
The second premise (if a law is not effective, it should not be a law) gives us a way to prove that something shouldn’t be a law. So if we can learn that gambling laws aren’t effective, we can prove that they shouldn’t be laws. The first premise establishes that gambling laws can’t be enforced. There’s a missing link between “can’t be enforced” and “not effective.” We should go to the answers looking for “If a law can’t be enforced, then it’s not effective.” Or, the contrapositive: “If a law is effective, it has to be enforceable.”
21.
Which one of the following, ██ ████████ ██████ ███ ██████████ ██████████ ██ ██ ████████ ██████
a
No effective law ██ ██████████████
This establishes that if a law is effective, it must NOT be UNenforceable. Take away the double negative and this means, if a law is effective it must be enforceable. This establishes that since gambling laws aren’t enforceable, they cannot be effective. 
b
All enforceable laws ███ ██████████
This means that if a law is enforceable, it’s effective. This reverses what we’re looking for. (B) doesn’t prove that if a law isn’t enforceable, it can’t be effective. In other words, (B) still allows for gambling laws to be effective, even though they’re not enforceable.
c
No legal prohibitions ███████ ████████ ███ ████████████
A premise already establishes that gambling prohibitions are not enforceable. (C) doesn’t establish that the gambling prohibitions are not EFFECTIVE. There’s a difference between “not enforceable” and “not effective.”
d
Most citizens must █████ ████ █ ███ ███ ███ ███ ██ ██ ██████████
The premises don’t give us enough to establish that most citizens don’t agree with gambling laws; so (D) doesn’t establish that gambling laws aren’t effective. We know that most citizens recognize that they’re not enforceable, but this doesn’t imply that they don’t agree with the laws.
e
Most citizens must █████ ████ █ ███ ███ ███ ███ ██ ██ ████████████
The premises already establish that gambling laws are not enforceable. (E) doesn’t establish that the laws are not EFFECTIVE. There’s a difference between “not enforceable” and “not effective.”