user: Question: What Young rider classification has a Stage of 18?
Columns: ['Stage', 'Winner', 'General classification', 'Points classification', 'Young rider classification', 'Team classification']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['P', 'Thierry Marie', 'Thierry Marie', 'Thierry Marie', 'Jesus Blanco Villar', 'Système U']
SQL:

assistant: SELECT "Young rider classification" FROM "Table" WHERE "Stage" = '18';