Define immortal. Do you mean "Cannot die naturally" or "Can't die, period"?
English
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I figure it'd be a moving target, eventually becoming very close to the latter. Assume that, for any given point in the time period we're talking about, people have the choice [I]not[/I] to die.
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The former is improbable. The latter is impossible.
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Jellyfish says "-blam!- IMPROBABLE".