user: Question: What is the track whent the vocal percussionist is unknown?
Columns: ['Track no.', 'Track', 'Original Artist', 'Soloist(s)', 'Vocal Percussionist', 'Arranger(s)']
Types: ['real', 'text', 'text', 'text', 'text', 'text']
Sample row: [1.0, 'Does Your Mother Know', 'ABBA', 'Nick Barstow', 'Danny Swift', 'Alexei Kalveks']
SQL:

assistant: SELECT "Track" FROM "Table" WHERE "Vocal Percussionist" = 'unknown';