grammar - What is the answer?

This might seem like a simple question, but I'm not sure it definitely is.

What is the Subject, in its most likely reading, of the question:

• What is the answer?

Is it the noun phrase (NP) What, or is it the noun phrase the answer?

How can we tell? In other words what evidence do we have?

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^{I reserve the right as stipulated in the guidance to ask questions that I know the answer to. I ask the question because a good answer would be useful to point to in other answers and because I think the question of evidence is interesting enough in its own right}