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It depends on the source. Like the OP implied, it's pretty easy to distinguish octaves on a piano or guitar because even single notes from those instruments are not really pure tones. Try it with a pure sine wave some time and you might find that you have a hard time distinguishing octaves.
By the way, Cm implies a triad (chord) made up of three tones "C - Eb - G" ... C# refers to a single tone.
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