which? where? how?
哪 centers on questioning: asking which, where, or how. It is a general interrogative marker used to request specific information.
哪 combines 口 (mouth) with 那, which likely contributes the sound. The 口 component suggests a spoken question, fitting the interrogative meaning.
The mouth 口 asks a question, and 那 provides the sound. Picture a mouth opening to ask 'which?' or 'where?'.
For ダ, imagine a person asking 'Duh, which one?' with a confused look: duh -> ダ, and the mouth 口 is asking the question.
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