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And you're bringing up exactly one of the core elements to this. Terminating the conversation is worse than saying "I don't know."
As for the expectation that you should know the answer, I'd say that is unrealistic. It is commonplace for interviewers to purposefully ask questions until you hit the boundary of your knowledge. They do this to get an idea of your breadth and depth of knowledge. As candidates though, we don't know that is what is happening and how much breadth or depth is appropriate.
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