Which of these is the distance between two notes in written music?

ledger line
interval
octave
tacet
16.
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Which of these are lines added above or below the staff that allow for higher or lower notes to be written?
tacets
intervals
ledger lines
octaves
17.
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Which of these means the selected notes should be played an octave above where they are written?
8av
8vb
8va
8ba
18.
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Which of these means the selected notes should be played an octave below where they are written?
8va
8vb
8ba
8av