Top Law Schools

I had a question about Q22 in Section 2 of Practice Test 37. I answered D (the correct answer is given as B) and I don't really see an immediately obvious problem with my reasoning. I said that "No law that cannot be enforced should be enacted", since the passage implied that it's impossible to tell whether a crime was motived by a "sincere desire" to achieve a greater good, and therefore it is impossible to enforce since there is no distinction between sincere desire and lack thereof in practice. Thoughts?

(D) is incorrect because the policy in question is not a law. Judges would not be "legally required" to mitigate punishment. They simply have the option of doing that now, but apparently shouldn't.

(B) is nice because the two opposing sentences "Granted, some criminals..." and "Nonetheless, judges..." are compared by the phrase "better to err..." in the solution in a manner consistent with the original statement.

Yes, I suppose you're right. Thanks.