user: Question: What unit of length is being used for the route with "Replaced by US 81" in their remarks section?
Columns: ['Route Name', 'Direction', 'Termini', 'Junctions', 'Length', 'Population Area', 'Remarks']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['SH 2', 'South North', 'Laredo, Texas La Salle County', 'none', 'miles (km)', 'Laredo Webb', 'Replaced by US 81']
SQL:

assistant: SELECT "Length" FROM "Table" WHERE "Remarks" = 'Replaced by US 81';