If there is a 10 CM ruler and someone ask you how long is that. The answer should be 10CM. If there is a 5 CM ruler and someone ask you how long is that. The answer should be 5CM. Now, If there isn’t any ruler and someone ask you how long is that. I should answer 0 or “N/A”? In this case, does 0 and “N/A” have the same meaning?
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More accurately we can say that the question does not arise. It is assumed that the questioner is clear about what ruler he is referring to and the answerer with a reasonable application of mind can decipher what ruler the questioner is referring to .The question will then arise and the answer can be expected.
One ambiguity can arise in answering the question. The questioner can say the ruler can exist with a “zero” length and if the question is about the length of the ruler it is correct to answer “zero”. N/A can also be used if instead of answering the question we say that the question does not beg an answer. Both of course are not the same as N/A would imply the answerer is challenging the correctness of the question itself while in the case of “zero” reply a technically correct answer is being given.