Effective October 1, 2023, the examinations will be referenced to the
2023 NEC Code Book
Helpful Tip #152
Don’t be fooled into picking “None of the above” answers just because you are not certain of the correct answer.
"None of the above" answer choice is usually a distractor used by test writers as an effective option for factual information in our case math questions to make this question more challenging. Hence, in test questions where the answer requires computation of some sort, you will more than likely see "none of the above" as the final option. This final option will confuse you and won't allow you to focus on a set of options that must contain the answer.
In a test-study, a 40-item general-knowledge multiple choice test was given to a number of undergraduate students. The test-study found that 45% of the time examinees selected 'none of the above' correctly, 19% selected a wrong answer, and 26% wrote no answer.
If you are not sure of the answer work backward from the answers to the question and plug in the information given. If after all that it still doesn't add up, then make your best selection, in some cases the correct answer will be 'none of the above'.